March 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
burning under layer brings about a surface explosion and a marked reduction in burning rate. The slower burning material settles back on the block surface and burns until the fresh block surface is reignited. The process is cyclic. This behavior was illustrated in the 5% charcoal block where charcoal and linseed oil were the two reducing agents. The two reducing agents need not be different chemical compounds. If the same reducing agent is present in two different conditions and one part reacts more readily than the other, the same periodic burning will result. This was illustrated by the 11% charcoal block containing potassium nitrate. The variation in burning rate was indicated by gas flow and by gas temperature. In general, surging was more pronounced with cast than with pressed mixtures. Cast mixtures containing sodium or potassium nitrate without any acid or water exhibited particularly violent surging. It has been shown that potassium nitrate augments differences in the reactivity of the charcoal particles. CHUFFING
h phenomenon similar to surging and known as chuffing has been observed in rocket fuels. The latter burn under pressures of thousands of pounds per square inch and this range is entirely dif-
565
ferent from that investigated here in connection with surging. Chuffing is more serious a t lower pressures. No attempt to relate surging and chuffig has been made. ACKNOWLEDGMENT
€I. F. Johnstone was technical director of the laboratory, and J. C. Bailar, Jr. was a project adviser. C. H. Simonson and D. G. Ehrlinger assisted with the gas analysis as well as the test procedure. Other members of the staff who contributed were R. Pat, Connor, R. C. Johnson, and P. N. Rylander. LITERATURE CITED
(1) Blayden, H. E., Riley, H. L., and Shaw, F., Fuel, 22,32,64(1943). (2) Kolthoff, I. M., Sandwell, E. B., and Muskovitz, B., J. Am. Chem. Soc., 55, 1454 (1933). (3) Lambert, J. D., Trans. Faraday Soc., 34,1080 (1938). (4) Parry, R. W.,and Comings, E. W., IND. ENO.CKEM.,42, 557 (1960). (5) Sihvonen,V.,Trans. Faraday Soc., 34,1062 (1938). (6) Strickland-Constable,R.F., Ibid., 34,1074 (1938). RhcEIverD January 3, 1949. Presented before the Division of Gas and Fuel Chemistry a t the 116th Meeting of the AMERICANCHEYICAL SOCIETY,Atlantic City, N. J.
CHEMISTRY OF WESTERN PINES Pine Knots and Paint Durability ARTHUR B. ANDERSON’ Western Pine Association Research Laboratory, Portland 2, Ore. Paint damage over pine knots in pine lumber usually occurs in definite stages of deterioration. The sequence of this paint blemish is the appearance of the familiar brown discoloration, followed by cracking, and finally peeling of the paint over the knot area. This damage to paint is attributed ohiefly to the unfavorable effects of the extractives present in the knot. This paper deals with the isolation and preliminary examination of the chemical
nature of the extractives present in pine knots from three commercially important western pines-ponderosa, Idaho white, and sugar. Experimental evidence is offered, for the first time, indicating which components in knot extract are largely responsible for paint discoloration over knots. The information was helpful in arriving at a knot-sealer formulation which has proved beneficial in alleviating this paint problem.
0
ponderosa pine sawdust was extracted with 95y0 ethanol, and the concentrated solution was tested for paint durability. The extract was reported to contain a rosin, chiefly abietic acid, and turpentine. The conclusion reached was that although the extractives affect the durability of paint coatings, sometimes favorably, sometimes unfavorably, the effect of such extractive substances is much less important in paint life than the physical structure of the wood. When white or pastel paints, enamels, or lacquers are applied over pine knots, paint damage over these knots usually occurs in definite stages of deterioration. The symptoms appear first as the familiar brown discoloration, followed by cracking, and finally peeling of the paint over the knot area (Figure I). This paint blemish, in most cases, manifests itself long before the clear portion of the board needs repainting. The damage to paint is attributed chiefly to unfavorable effects of the extractives, often indiscriminately called “resin” or “pitch,” present in the knot. This laboratory has been conducting extensive research work on the development of a satisfactory knot sealer to solve this problem. The investigation reported here deals with the quantity and nature of the extractives in ponderosa, sugar, and Idaho white pine knots and was undertaken to obtain a better understanding of the cause and nature of this type paint damage.
VER 2 billion board feet of knotty pine boards are pro-
duced annually in the manufacture of lumber from three commercially important western pines, ponderosa (Pinus ponderosa), Idaho white (Pinus monticola), and sugar (Pinus lambertiana). Knots, which are derived from the branches in the bole of the tree, show an almost endless variation in size, color, form, and quality (8). Because of this wide latitude in the nature of knots in boards, five grades have been established for knotty pine lumber-namely, Nos. 1 to 5 Common, inclusive; each grade has specifically recommended end uses (12). Nos. 1 and 2 Common lumber contain, in the main, intergrown knots, so-called because the growth rings of the branch are intergrown or joined to those in the surrounding wood. Although often referred to as intergrown or live knots, the term “red knot” has wider usage among lumbermen. Such lumber is used, for instance, for knotty pine paneling, interior trim, furniture, and exterior siding. Since much of this lumber is painted, it is this type of knot which is the subject of this study, particularly to ascertain the nature and effect of knot extractives on paint ,films. The effect of ponderosa pine extractives on the durability of ;paint coatings has been reported by Browne (4). In that study, 1
Present address, 4622 S.E. River Drive, Portland 22, Ore.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I.
QUANTITY AND APPROXIM.4TE COAlPOSITION O F K S O T
Y-NaOH Soluble, %Petroleum Petroleum ether ether soluble insoluble Resin Phenol- Resin Fatty Phenolacids 10s acids acids ics 38.9 7.8 5.2 2.1 22.2 0.7 5.2 3.7 20.0 45.6 1.3 9.6 1.9 6.5 62.7
Vol. 42, No. 3
EXTR.\CTI\-ES
7
% Extract Rangea
Knot Species Ponderosapine 38.4-42.9 Sugar pine 24.1-37.1 Idaho white pine 28,5-40,4 a hfoisture-free basis.
EXTRACTION AND ANALYSIS
Intergrowi, or red knots, were sawed from kiln-dried ponderosa, sugar, and Idaho white pine lumber, and the joined bole growth rings were removed with a wood chisel and discarded. The segregated knots were chipped and ground in a Wiley mill to pass a 2-mm. screen. Ten to fifteen grams of each sample were weighed, in duplicate, and extracted with C.P. acetone for 8 hours; the quantity of extract was determined as described in a previous paper ( 2 ) . The extract from each of the three knot species \vas analyzed for petroleum ether, soluble and insoluble resin acids, fatty acids, and phenolic materials, together with steam volatiles, esters, and unsaponifiables. The procedure of analysis used was as follows: The acetone from an aliquot containing 7 to 10 grams of knot extract was distilled from a water bath. The extract was taken up in ethyl ether and transferred to a separatory funnel and washed several times with water to remove small quantities of water-soluble substances, if present. The ether solution was then thoroughly extracted with 3% sodium hydroxide solution to remove all acid-reacting materials. Dilute hydrochloric acid (5%) was added to the alkaline solution to precipitate the acidic fraction and the latter was recovered by extraction with ether. The ether solution was transferred to a separatory funnel and washed with 10% aqueous sodium chloride until neutral to Congo red, and the ether was removed on a water bath. The residue was separated into petroleum ether-soluble and -insoluble fractionsthat is, oxidized materials-by refluxing with 10 parts by weight of petroleum ether (boiling point 30" to 75" C.) for 1 hour, cooling, and decanting. This was iepeated twice and the petroleum ether solutions combined. The petroleum ether-insoluble residue was dissolved in 3 parts by weight of acetone to bT-hich there was added a slight excess of 50:50 solution (by weight) of cgclohexylamine (lfonsanto Chemical Company) in acetone to quantitatively precipitate the resin acids as the very insoluble cyclohexylamine salts ( 6 ) . The mixture was thoroughly agitated and set aside in an ice box overnight. The salts were then filtered and m-ashed carefully with cool, fresh acetone until free of nonresin acid entities. The salts Tvere transferred to a separatory funnel containing ether and were shaken vigorously with an excess of 5% hydrochloric acid until the amine salt crystals had disappeared. The dilute aqueous acid solution rvas re-extracted twice with ether and the ether solutions combined. The total ether solution was washed free of hydrochloric acid with 10% sodium chloride, followed with water. The washed ether solution was transferred to a tared flask, and ether was removed by distillation on a water bath. The flask containing the resin acids was dried in an oven a t 105O C. to constant weight and weighed as resin acids. The filtrate and washings from the resin acid amine salts were combined and acidified with dilute hydrochloric acid, and the acetone was removed by distillation. This acidic residue was dissolved in ethyl ether and transferred to a separatory funnel and washed free of mineral acid as already described. The ether solution was thoroughly extracted with 3% sodium carbonate solution to remove the nonresin acid fraction. This alkaline solution was acidified with dilute hydrochloric acid and the precipitate extracted with ether and washed free of mineral acid; the ether was removed and residue dried to constant weight. This was weighed as fatty acids. It contained, among other products, carboxylic acids, readily esterified with ethanol plus sulfuric acid a t room temperature, which suggest fatty acids. The ether solution, which was extracted with sodium carbonate, was washed with water until free of alkali; the ether was removed and the residue dried to constant weight. This was weighed as the phenolic fraction. A portion of this residue, when converted to the sodium salt, reacted with chloroacetic acid to
S e u r r a l s , $6 ____--____ 1-ola-
tile 10.0
5.6
3.0
Esters 3.7 9.4 7.2
Unsaponifiahle 10.1
9 8 7.8
form the corresponding ether derivatives, which would suggest phenolic compounds. However, this entity also appears to b e complex in nature and for purposes of this report will be designated as phenolic. Further characterization of this fraction is being investigated, as are all components of these various knot extracts. The petroleurn ether-soluble portion of the sodium hydroxidesoluble fraction \vas analyzed for resin acids and phenolics as described. After the precipitation of resin acids, only traces of sodium carbonate-soluble components were found in this fiaction. The esters, unsaponifiables, and volatile terpenes in the neutral or sodium hydroxide-insoluble fraction were determined according to the procedure reported previously ( 2 ) . The quantity and composition of the acetone-soluble knot extracts are summarized in Table I. QUANTITY AND NATURE OF EXTRACTIVES
Since knots contain from 24 to 43% extract, this entit>yis no longer among the minor constituent's, as is usually the case, in the average sapwood a,nd heartwood from so-called resinous wood ( I ) . This has also been pointed out by Jayme and Blischnok, who reported t,hat pine knots contained from 29 t,o 30% methanoibenzene-soluble extract, in contrast to the heartivood which contained approximately 67, extract ( 7 ) . Extractives of ponderosa intergrorvn knots of various diameters indicated that extractjive contents vere of the same order, irrespective of size. IloTnws, this does not appear to be true for Idaho and sugar pine knots. The larger knots (over 1.5 inches in diameter) of these two species contained less extract--that is, from 24 to 35%-than the smaller knots, which contained from 30 to 40% extract. The ceiiters of the larger knots, which are usually lighter in color than the rest of the knot area, contained less extract than the outer portion, or peripheral area of the knot. The predominating component' in ponderosa pine knot extract is the petroleuni ether-soluble resin acids, whereas this constituent appears to be among the lesser components in sugar and Idaho white pine knot extracts. On the other hand, Idaho white pine knot extract consists largely of petroleum etherinsoluble phenolics. This fraction, likewise, predominates in sugar pine, but not in the quantit'ies found in Idaho pine knots. Sugar pine knots contain approximately twice as much phenolics as ponderosa knot extract. There are other self-evident differences in percentage composition between each of these extracts. KVOT EXTRACT AND ITS EFFECT OV PAINT DL-RIRICI'I'Y
I n the discoloration of paint over pine knots, it has been reported that there is likely to be more yellow discoloration of the paint over knots nrhere sunshine is weakest (Q),and while the discoloration first manifests itself usually on the knots exposed to sunlight, the color tends to bleach. It has long been observed, however, that paint discoloration over ponderosa, sugar, and Idaho white pine knots is more prevalent over knots exposed to sunlight, but this discoloration does not tend to bleach in the presence of continued sunshine, rather it tends to become more intense. It is suggested that this apparent difference in behavoir of paint over knots may be attributed to the nature of the chemi-
March 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
dividual knot test panels, 2 6 / ~X B1/2 X l / 4 inches, were arranged in an enclosed circular rack of 47 inches diameter with the lamp in the center. A number of these painted knot panels were exposed directly to the lamp rays, and a like number of matched knot panels were similarly placed in the rack enclosed in amber glass containers to filter out the ultraviolet. The paint over the knot area on the panels exposed directly to ultraviolet discolored much more rapidly than the painted knot areas which did not receive the actinic rays, although the temperature (maximum 125' F.) in each case was the same. A preliminary study was made to determine which entity in the knot extract was largely responsible for the paint discoloration. For this purpose, ponderosa pine knot extract was first separated into its acidic fraction (sodium hydroxide-soluble) and neutral fraction, which contains the volatile, ester, and unsaponifiable components. The neutral fraction was a lightyellow viscous liquid, and the acidic fraction was a dark brown amorphous solid. This latter fraction %'as extracted with petroleum ether (boiling point 30' to 75' C.) t o separate the oxidized from the unoxidized acidic constituents. The petroleum ethersoluble fraction was a light yellow semisolid material, which, on standing, slowly deposited white crystalline resin acids. The petroleum ether-insoluble portion contained the highly tinctorial material, or color bodies, which was dark red to brown and amorphous. Two grams of each of these three fractions and two grams of the ponderosa knot extract were mixed with 18 grams of white lead-in-oil paint, making a 10% concentration, by weight, of each extract component in paint. Each of these was stripped on a ponderosa pine sapwood panel (Figure 2). Strip 1 is the blank of white lead-in-oil; strip 2 contains total knot extract; strip 3 consists of the neutral fraction; strip 4 is the petroleum ether-
Figure 1. Damage to Paint over a Pine Knot 'Unpainted (left), early damage (center), ultimate damage (right)
cal components present in the extractives of the various knot species. In the three western pine knots studied, sunlight is known to be an accelerating factor in paint discoloration over knotted areas. This accelerated discoloration rate has been indiscriminately attributed to both the infrared rays and the ultraviolet wave length. However, which wave length from the sun is largely responsible for the accelerated discoloration action appears not to have been established. It is known that the action of light on organic compounds depends a great deal on the wave length of the radiation used, and broadly speaking the shorter the wave length, the more drastic is the decomposition of the organic molecule (5). This suggests that ultraviolet light may be the contributing factor in facilitating the discoloration rate. To corroborate this, freshly painted (white lead-in-oil) knot test panels were placed in an ultraviolet ray circular cabinet using a Westinghouse 400-watt mercury vapor, Type H-I lamp as the light source. The in-
3
2
1
Figure 2.
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4
5
Knot Extract Entities in White Lead-in-Oil after Ultraviolet Exposure
1 = White lead-in-oil only; 2 = whole knot extract; 3 = neutral fraction; 4 = petroleum ether-soluble acidic fraction; 5 = petroleum ether-
insoluble acidic fraction
1
2
Figure 3.
3
4
5
6
Ponderosa Pine Knot Extract Components in White Lacquer Upper panel, room exposure; lower panel, ultraviolet exposure
7
8
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Vol. 42, No. 3
INDUSTRIAL AND ENGINEERING CHEMISTRY
1
2
3
Figure 4.
1
6
5
8
7
Sugar Pine Knot Extract Components i n White Lacqucr
Upper panel, room exposure; lower panel, ultraviolet exposure
!
__
..
I
2
Figure 5 .
3
4
5
6
7
.. ..
8
Idaho White Pine Knot Extract Components i n White Lacquer Upper panel, room exposure; lower panel, ultraviolet exposure
soluble acidic fraction; and strip 5 contains the petroleum etherinsoluble acidic material. Strip 2 gave a pale tan color, strips 3 and 4 remained practically white, and strip 5 gave a tan color slightly darker than strip 2. The panel was exposed to ultraviolet for 168 hours and, although there was but a slight change in the color of strips 3 and 4, strips 2 and 5 had darkened, particularly the latter. This would indicate that the petroleum etherinsoluble acidic fraction (oxidized portion) is largely responsible for the familiar brown discoloration over painted ponderosa pine knots. The petroleum ether-insoluble fractions from each of thc three knot extracts were separated into resin acids, fatty acids, and phenolics, according to the procedure already described, except that each entity, as isolated, was dried under vacuum at 50" to 60' C., to minimize further oxidation. The petroleum ether-soluble fractions were separated into resin acids and phenolics for paint discoloration tests. Each of these five entities, together with the whole knot extract and neutral fraction, was mixed with white pigmented nitrocellulose lacquer, instead of the white lead-in-oil. Here, the colors were a little more intense, partly because of the greater solubility of these knot extract components in lacquer solvents. Fifteen per cent by weight of each of these components in white lacquer was prepared and stripped on wood panels, in duplicate (Figures 3, 4, and 5 ) . The numbered strips in each panel consist of the following: strip No. 1 2 3 4 6
6
7
8
Extract Component Control, no extract Whole knot extract Total neutral fraction Petroleum ether-soluble resin acids Petroleum ether-soluble phenolics Petroleum ether-insoluble fatty acids Petroleum ether-insoluble resin acids Petroleum ether-insoluble phenolics
The upper panel in each caw --as room-exposed, and the Io~ver panel was exposed to ultraviolet light for 168 hours. The approximate colors of room-exposed and ultraviolet panels are givcn in Table 11. While it is difficult to visualize the actual degrees and shadings of color and color changes that have occurred, from either the figures or descriptions givcn in Table 11, the following conclusions are indicated: 1. The color intensities of the whole knot extract (strip 2 ) rere: (1) Idaho white, (2) sugar pine, and (3) ponderosa pine This observation was also made in the field-that is, the Idaho white pine knots discolor paint more readily than sugar pine, and sugar pine knots seem to discolor paint more readily than ponderosa knots. 2. The slight color imparted by the neutral fractions (strip 3) is probably due to traces of coloring substances not complete11 removed in the entity separation. 3. It is evident that the petroleum ether-soluble resin acids (strip 4 ) do not discolor paint coatings. 4. The petroleum ether-soluble phenolics (strip 5) become colored on exposure to ultraviolet. 5. The petroleum ether-insoluble fatty acids (strip 6) are largely responsible for the immediate discoloration of paint ovcr knots. However, on exposure to ultraviolet this component lightens in color, or tends to bleach. 6. The petroleum ether-insoluble resin acids (strip 7), may contribute slightly to the immediate discolor of painted knots, but they become practically colorless when exposed to ultraviolet. 7. The petroleum ether-insoluble phenolic fractions (strip 8) impart some immediate discoloration to paint films, which becomes much darker on expnsuir to ultraviolet. Thus, the petroleum ether-insoluble fatty acids in thew knot extracts are largely responsible for the immediate discoloration of paints over pine knots; the petroleum ether-insoluble phenolics contribute to a lesser degice to immediate discoloration; and
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLB 11. APPROXIMATECOLOROF LACQUER IMPARTED BY KNOTEXTRACT COXPONENTS Strip 1 Knot Extract Control Ponderosa Room exposed White Ultraviolet exposed White Sugar Pine Room exposed White White Ultraviolet exposed Idaho White Room exposed White White Ultraviolet exposed Lighter shades of respective colors. b Darker shades of respective colors.
+-
Strip 2 Whole Knot Extract
Strip 3 Neutral
Ivory Cream
Ivory - Cream -
Cream Tan
Ivory Cream
Cream Tan + b
Ivory Cream
-
-
the phenolics are largely responsible for the accelerated discoloration of paint over pine knots when exposed to ultraviolet or sunlight. Apparently, after the knot extract is absorbed into the paint film, the amorphous constituents impart a brittleness to the paint film, with subsequent cracking and checking, and finally peeling of the paint from the knot surface. APPLICATIONS OF RESULTS
Painters have recommended the use of shellac or powdered aluminum in spar varnish as a sealing coat over knots to prevent damage to paint. It has also been prescribed that all pitch should be scraped, burned, or wiped (with turpentine) from knots prior to painting (IO). It is well known, however, that although the recommended sealer and proposed methods of pitch removal will retard paint discoloration or damage, they are not entirely satisfactory because the paint over the knot so treated generally will fail in a shorter time than the normal life of the paint. The failurc of shellac or available aluminum paints as knot sealers is due, in part, to the limited ability of these products to form a protective film over the knot surface which will remain impervious to the knot extract. Also, shellac will not remain intact with the wood, particularly when subjected to outside exposure (11). It is reported that these shortcomings of shellac can be minimized by plasticizing the shellac with blown castor oil, which serves to make the shellac more nearly compatible with paint. Even after scraping, burning the knot area, or washing the knot with a solvent, a sufficient quantity of extract usually remains at or near the surface of the knot to cause subsequent paint discoloration. The requirements for a satisfactory knot sealer are, in part, revealed by the quantity and nature of the knot extract. Since pine knots contain large quantities of extractives consisting of a complex mixture of organic compounds, it is not surprising that paints, lacquers, and enamels become blemished over knot areas because of the mutual solubility of these several types of protective coatings and the various types of organic materials present in the knot extract. One of the primary requirements, therefore, for an effective sealer, is that it shall deposit a film over the knot area which will remain impervious to the extract. Further, the sealer must adhere firmly and not break away from the knot area. The sealer should be conipatible with paints, and
Petroleum Ether Soluble Strip 4 resin Strip 5 acids phenolic White White Ivory Ivory Ivory White
+
--
--
, Tan
-
Tan Cream
+-
Tan Tan
+-
Ivory White
+-
Tan Tan
Ivory Tan
--
Ivory Tan
Tan
Petroleum Ether Insoluble Strip 7 resin Strip 8 acids phenolic
Ivory Cream+
-
- -
Strip 6 fatt aci&
++
Ivory White
--
Ivory Tan
++ +
+
Cream Tan
--
Tan Tan
+
+
++
finally, it must maintain these requirements under all weathering conditions. Laboratory accelerated paint tests, such as ultraviolet and weatherometer exposures, have indicated that a formulation consisting of 10 parts phenolic resin varnish (BV 9700, Bakelite Corporation) and 1 part polyvinyl butyral (XYHL, Carbide and Carbon Corporation) with 19 parts by weight of ethanol as a solvent, is one of the better sealers found thus far (3). This sealer has been widely used commercially for the past 4 years on western pine lumber and the results thus far indicate that it performs quite satisfactorily as a knot sealer. However, it must be pointed out, that while a sealer may be satisfactory for knots of several species, it does not necessarily follow that the same scaler will perform equally well on all knot species. This apparent variation in knot sealer durability is attributed to the varying quantity and, in particular, to the chemical nature of the extractive components present in each knot species. ACKNOWLEDGMENT
The author wishes to acknowledge the assistance given him by E. W. Loaier and to thank the Western Pine Association for permission to publish this work. Also, the cooperation of the Monsanto Chemical Company, Seattle, Wash., in carrying out the weatherometer tests is gratefully acknowledged. LITERATURE CITED
Anderson, IND.ENG.CHEM.,36, 662-3 (1944). Ibid., 38,560 (1946). Anderson, Western Pine Assoc., Lab. Note No. 21 (Revised May 15,1947). Browne, U. 9. DeDt. Aar. Forest Products Lab.. MimeonraDh R-1073(October 19355. Dahar, “The Chemical Action of Light,” p, 289, London, Blackie and Son, Ltd., 1931. Harris, U. S. Patent 2,419,211 (April 22, 1947). Jayme and Blisohnok, Holz Roh- u. Werkstof, I, No. 14, 538
- _
(1938).
Rapraeger, The Timberman, 39, No. 10, 16 (1938). Walker and Hickson, “Paint Manual,” rept. BMS 105, p. 73, Washington, D. C., U. S. Dept. of Commerce (Oct. 11, 1945). Ibid.,p 76. Ibid..D. 128. Westein Pine Assoo., Standard Grading Rules, pp. 15-22 (April 15, 1947).
.
RECEIVED October 29, 1948.
