IRVING S. GOLDSTEIN, WILLIAM A. DREHER, EDWARD B. JEROSKI, J. F. NIELSON, WILLIAM J. OBERLW, and J. W. WEAVER Research Department, Koppers Co., Inc., Verona, Pa.
Wood Processing Inhibiting against Swelling and Decay
The swelling and shrinking of wood with changes in moisture content can be minimized by bulking with resins or by chemical modification involving the formation of cellulose ethers or esters. Such chemically modified woods will not rot
0
F THE three effective methods for stabilizing the dimensions of woodnamely, bulking, replacement of hydroxyl by less polar groups, and cross linking ( 73)-only bulking has attained commercial significance, particularly in resin impregnated die model lumber (77). T h e bulking effect of resins is shown by a correlation between the methylolphenols content of aging phenolformaldehyde resin solutions and their ability to stabilize the dimensions of wood (70). When wood is chemically modified by formation of ethers or esters of cellulose, the swelling and shrinking are reduced in accordance with replacement of hydroxyls and bulking, as the groups introduced are much larger than the modified hydroxyl. Acetylation of wood (74, 76) provides a high degree of dimensional stability without reducing its toughness. Other methods of chemical modification of wood were investigated. T h e cyanoethylation of cotton textiles has been reported ( 2 , 4, and wool ( 9 )and cotton (3)have been treated with P-propiolactone. By modifying these two methods the dimensions were stabilized and the decay was prevented for pine wood. Wood and P-propiolactone react readily without a catalyst to give products which probably contain polyester side chains made u p of carboxyethyl groups grafted to the cellulose backbone. T h e wood is permanently swollen by the treatment and dimensionally stabilized in proportion to the permanent swelling and weight gain.
Wood can be cyanoethylated with acrylonitrile to give products with similar properties but shorter side chain substituents. These treated woods absorb as little as 33% as much moisture and swell tangentially only 25 to 30% as much as untreated samples. The pickup of liquid water and the attendant swelling are similarly reduced. T h e propiolactone treatment does not lower the strength of the wood, while cyanoethylation lowers the impact strength. Treated woods, even after simulated weathering, resist the attack of wood-destroying fungi. Good dimensional stability can be imparted to wood by treatment with aged solutions of phenol-formaldehyde resins. T h e appearance and subsequent disappearance of the methylolphenols in the solution can be correlated with ita ability to impart this stability. Complete stabilization of wood by bulking or by replacement of the cellulosic hydroxyls is not possible, since the small water molecules can always reach regions of the wood structure inaccessible to the bulking agent or substituting group. 0-Propiolactone Treatment
Experimental. 8-Propiolactone reacts with alcohols with or without acid or basic catalysts (8). 8-Propiolactone and wood react readily without a catalyst to give a dimensionally stable product with no loss in toughness. Concentrated
solutions distort wood by causing a high degree of swelling which is accompanied by delamination and splitting. To bring about moderate treatments which d o not cause excessive swelling and loss of strength, it is necessary to use dilute solutions of the 0-propiolactone in inert solvents such as ketones, ethers, esters, or hydrocarbons. This reaction can be controlled by the concentration of the lactone in the solvent. The wood is impregnated with the solution by a vacuum-pressure process to ensure complete and intimate contact. While the wood is submerged the solution is heated. T h e temperature of heating is limited by the boiling point of the solvent a t the pressure used, and the duration of heating required for a given degree of reaction is obviously less for higher boiling solvents. The wood is then oven dried. The extent of reaction may be measured by the gain in weight of the treated wood. However, reaction paths with cellulose are multiple and include etherification, polyesterification, and selfpolymerization of the lactone ( 3 ) . About one-third of the apparent weight increase of southern yellow pine (shortleaf, P. echinata) treated in the form of 3/4-inch cubes may be ascribed to self-polymerization of the lactone. One such cube treated to an apparent weight increase of 4oy0 showed only 3oy0 weight increase after leaching with water and a further reduction in the net increase in weight to 24% after acetone extraction. Two blocks treated with the lactone in acetone to a n apVOL. 51, NO. 10
OCTOBER 1959
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parent weight increase of 15% showed a net increase in weight of only 10yc after extraction Lvith acetone. T h e etherification of the hydroxyl groups of cellulose produces carboxyerhylcellulose \vhich can react with more @-propiolactone to form polyester side chains made up of carboxyethyl groups grafted to the cellulose backbone. C:ellO(CHICH,COO),CHIC:H,C:OOH
The degree of this polyesterification reaction has been estimated by measurine; the ability of the lactone treated \vood to bind copper. Southern yellow pine cubes were treated with solutions Lvith varying concentrations of 13-propiolactone in acetone under reflux for 20 hours. The treated blocks were then extracted lvith acetone. dried and weighed, impregnated with a cupric acetate solution, leached lvith water for 6 days until no copper could be detected in the Lvashings. dried, and analyzed for copper. Dimensional stability was measured on ponderosa pine (Pinus ponderosa) specimens, about 10 inches tangentially, 11/4 inches radially, and inch longitudinally. oven-dried a t 105’ C . and then weighed to 0.01 gram and measured to 0.001 inch tangentially by a jig and dial gage. Sets of these samples were treated rvith solutions containing various concentrations of 8-propiolactone in acetone. The difference betiveen the dry treated weight and the original dry weight is the net weight gain, while the increase in tangential length is called sivelling due to treatment. The treated samples were then allowed to come to equilibrium with an atmosphere at 80’ F. and 707c relative humidity. The weight
Figure 1. Equilibrium moisture content of chemically modified pines in a moist
gained during equilibration represents equilibrium moisture content. The reduction in swelling is obtained by comparison with untreated controls in the same environment. The samples were then saturated with water and similar calculations were made. As the properties of Ivood most affected by stabilization with bulking resins are hardness, which is ,increased, and toughness, which is decreased, related properties of kvood treated with 8propiolactone were measured. Southern yellow pine specimens used for toughness measurement by impact strength were 2 X 2 X 5 inches. They were broken alternately in the radial and tangential directions on a Tinius Olsen plastics impact tester using the Charpy anvil (simple beam) and a four-inch span. Crushing strength \vas measured on flat grained samples X 1 X 3 inches. Half the specimens were impregnated with water before compression perpendicular-to-grain tests kvere performed. Load was applied on a radial surface by an Instron testing machine. The resistance to decay by ivooddestroying fungi was evaluated by the soil block test ( 7 ) using a modified weathering schedule. Blocks of southern yellow pine (shortleaf. P. echinata). 3/4-in cube, were treated with 8-propiolactone to give three groups Lvith different weight gains. Half the blocks were weathered on a device submerged in running water for 30 minutes of a 72-minute cycle and then exposed to ultraviolet radiation Lvhile drying in a stream of air for the remaining 60% of the time. After one month of weathering the samples \vere dried. equilibrated, and lveighed, and then weathered and unweathered samples
atmosphere indicates that lactone is less efficient because of the longer side chains grafted to the cellulose
Figure 2. Reduction in swelling of chemically modified pines in moist atmosphere shows that lactone is less efficient, but the swelling has been substantially reduced b y both treatments
A.
A.
Ponderosa pine treated with 6-propiolactone 6. Cyanoethylated ponderosa and southern yellow pines
1 3 14
Ponderosa pine treated with p-propiolactone B. Cyanoethylated ponderosa and southern yellow pines
INDUSTRIAL AND ENGINEERING CHEMISTRY
were exposed in duplicate for 3 months to four species of wood-destroying fungi according to the ASTM soil-block method. Results. Table I gives clues io ihr reaction product. If the assumption is made that none of the copper forms cross links. the ratio of lactone to copper gives a direct measurr of the average length of the polyester chains. At the other extreme. if all the copper is assumed to cross link, the po1yrstc.r chain length would be half the ratici given in the table. Some intermediate value is probably the true one. The data do sho\v that polyesterification is the principal reaction. and that as the amount of ~-propiolactonereactcd Fvith the wood increases the number of terminal carboxyl groups capable of binding copper increases at a s l o \ \ ~ r rate. Polyesterification is even more important at higher \-deto phenol \vas 1.5 t o I . Portions of the solution n ~ r eaged at 4 ' : 23'. and 38" C. for cstcrided periods and \verr sampled at convenient rimes for chrrnical and physical analyses and for trciitinent of wood. The six methylolphenols tvcrc idcntified in the resin solution. T'hr). \vert' separated by paper chromatography (5, 6) and developed \vith Gibbs' reagent 10.04 gram of 2,G-dibromo-,Ychloro-p-quinone imine in 100 mi. of 9.5% ethanol), Vndevelopcd strips Ivrre cut into sections to inatch the dcveloprd spots and eluted \vith Tvatrr \l:ater extracts \\.ere made up to volurne containing 0.04fr: 4-arniiioantii,yriric, 0.08C: potassium ferriqanide, and 0.7$;, a~rinioniuni hydroxide, and their absorbanccs compared ~ v i t hstandard solutions a t 52.5 mp. The error in most cases appeared to be 10ycor Irss. Ponderosa pine dimension M.afcrs \\.ere impregnated with rrsin solutions at various ages, then cured in an oven a t 105' C. for 24 hours. Results. Table L7 shows the coinlmition of a nominal 30Yb resin solution during aging at 23' C:. Analyses ivere discontinued after 133 da).s because of the disappearance of the rnetliylolphenols. T h e solution srparatcd into t i v o phases afrer 15- days. The total m e t h ~ ~ l o l ~ ~ t ~ ereached riols a platcau bet\\*een 10 and 60 days. The dicrease afrer this time indicates the formation of more hiShly condensed molecules. Figure .5 shinvs the relationship observed benveen the age of the solution used to impregnate ponderosa pine
WOOD PROCESSING Table V.
The Methylolphenols Occurring in a Phenol-Formaldehyde Resin Solution Can Be Separated by Chromatography Moleso of Compound in 1000 G . of Solution Age of 0P 2,6-Di- 2,4-DiDiphenyl- 2,4,6-TriTotal Solution, Methylol- Methylol- methylol- methylol- methane niethylol- methylolphenol phenol Derivative” phenol phenols6 phenol Days phenol 0 0 0
0 0 0.23
0
0 0.18
0.26 0.34 0.26
1.41 2.16 2.18
0.10 0.18 0.28
0.36 0.38 0.31
2.31 2.15 2.12
0.36 0.31 0.32
0.37 0.64 0.59
0.37 0.37 0.35
2.19 2.42 2.16
0.25 0.21 0.18
0.24 0.20 0.17
0.42 0.35 0.31
0.11 0.17 0.15
1.47 1.30 1.10
0.11 0.09
0.14 0.10
0.12 0.08
0.24 0.22
0.12 0.04
0.88 0.63
0.08 0.06
0.09 0.07
0.08 0.07
a. 17 0.12
0.04 0.02
0.55 0.40
0
0
1 2
0 0.30 0.49
0.23 0.34
0 0.33 0.21
0.15 0.22
3 4 6
0.50 0.92 0.84
0.27 0.30 0.35
0.16 0.28 0.22
0.22 0.32 0.33
9 30 49
0.70 0.51 0.47
0.50 0.41 0.39
0.21 0.14 0.23
0.44 0.53 0.44
63 77 84
0.44 0.39 0.28
0.34 0.30 0.27
0.31 0.41 0.35
91 98 106
0.24 0.21 0.16
0.21 0.16 0.13
113 119
0.15 0.10
126 135
0.09 0.06
0
0
1.01 1.49
Moles of diphenylmethane derivative, 3,3’,5,5‘-tetraniethyl014,4’-dihydroxydiphenylmethane, have been multiplied by two (equivalents) as it is formed from two moles of the trimethylolphenol. Original concentration of phenol was 2.69. Total values include only the six compounds determined. TWOother compounds in the aged solutions were not identified.
T h e appearance and subsequent disappearance of the methylolphenols in a phenol-formaldehyde resin solution can be correlated with the solution’s ability to impart dimensional stability to impregnated wood. T h e rate a t which these methylolphenols appear and disappear can be controlled by the temperature a t which the solution is aged. T h e decrease in dimension stabilizing power as the methylolphenols disappear is more marked in a high humidity atmosphere than in liquid water. However, even this effect is relatively small compared to the residual dimension stabilizing ability of the solution. AS long as the resin is water soluble good stabilization can be obtained. This effectiveness in the larger capillaries accessible to the resin molecules is enhanced when the simple methylolphenols are present, by their ability to enter still finer capillary structures. Complete stabilization of the wood is not possible as water can always reach regions of the wood structure inaccessible to the smallest methylolphenol molecules. literature Cited
dimension wafers and the amount of resin remaining in the wood after curing. When these resin treated dimension wafers were allowed to come to equilibrium with a n atmosphere a t 80’ F. and 70y0 relative humidity, their swelling, compared to untreated controls, is reduced (Figure 6). As monomethylolphenols are formed in the solution the dimensional stabilization, particularly against high humidity, becomes good despite low resin yields. The increase to a maximum of dimensional stabilization appears to parallel the increase to a maximum of the total concentration of simple methylolphenols. When the concentration of methylolphenols starts to decrease, the dimensional stabilization
does also, but a t a much slower rate. Although larger resin molecules are not as effective as the simple methylolphenols in entering the fine capillary structure of the wood they are still effective in the larger capillaries which contribute to a large extent to the swelling of untreated wood. When the resin solution was aged a t 38’ C. it separated into two phases after only 23 days, whereas aging a t 4’ C. was abandoned after 300 days with no evidence of separation. Pine dimension wafers treated with all three of these solutions were impregnated wtih liquid water and the reductions in swelling were measured. Figure 7 clearly shows the short service life of the solution aged a t 38’ C.
(1) Am. SOC. Testing Materials, Philadelphia, Pa. “ASTM Standards,” 1956 Supplement, part 4 , p. 142, Tentative Method D1413-56 T. (2) Compton, J., Martin, W . H., Ward, B. H., Jr., Thompson, D. D., Textile Znds. 117, 138A-D, 188 (1953). (3) Daul, G. C., Reinhardt, R. M., Reid, J. D., Textile Research J . 24, 738-47 (1954); 25, 330-3 (1955). (4) Zbid., 25, 246-53 (1955). (5) Freeman, J. H., Anal. Chem. 24, 9559, 2001-2 (1952). ( 6 )74,6257-62 Freeman. (1952): J. H.. J . Am. Chem. SOC.
.,
(7) Freeman, J. H., Lewis, C. W., Ibid., 76, 2080-7 (1954). (8) Gresham, T. L., Jansen, J. E., Shaver, F. W., Gregory, J. T., Beears, W. L., Zbid., 70, 1004-6 (1948). (9) Jones, H. W., Lundgren, H. P., U.S. Patent 2,517,573 (Aug. 9, 1950); Textile Research J. 21, 620-34 (1951). (10) Seborg, R. M., Tarkow, Harold, Forest Prods. J . 7, 256-61 (1957). (11) Seborg, R. M., Vallier, .4.E., J . Forest Products Research SOG. 4, 305-12 (1954). .,. \ . _ _
(12) Sprengling, G. R., Freeman, J. H., J . Am. Chem. SOC.72,1982-5 (1950). (13) Stamm, A. J. “Surface Properties of
Cellulosic Materials” in “Wood Chemistry” (L. E. Wise, E. C. Jahn, eds.), 2 n d ed., p. 747, Reinhold, New York,
k0C OI S O L V T I O N , O N S
E
Figure 6. Reduction in swelling of ponderosa pine impregnated with a phenol-formaldehyde resin solution aged at 23’ C. shows that stabilizing efficiency decreases with increasing sage of the solution
Figure 7. Reduction in swelling in water of ponderosa pine treated with three phenol-formaldehyde resins aged a t different temperatures shows that higher temperatures drastically reduce the service life of these resins A.
38’ C.
B.
23’ C.
c.
do c.
1952. (14) Stamm, A. J., Tarkow, H., J. Phys. B Colloid Chem. 51, 493 (1947). (15) Tarkow, H., Stamm, A. J., J . Forest Products Research SOG.3, 33-57 (1953). (16) Tarkow, H., Stamm, A. J., Erickson, E. C. O., “Acetylated Wood,” Forest Products Lab. Mimeo 1593 (1946).
RECEIVED for review September 29, 1958 ACCEPTED May 21, 1959 Division of Cellulose Chemistry, 134th Meeting, ACS, Chicago, Ill., September 1958. VOL. 51, NO. 10
OCTOBER 1959
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