ACETYLATED WOOD 

Information Reviewed and Reaffirmed 

November 1953 

This Report is One of a Series 

Issued in Cooperation with the 

AIR FORCE-NAVY-CIVIL SUBCOMMITTEE 

on 

AIRCRAFT DESIGN CRITERIA 

Under the Supervision of the 

AIRCRAFT COMMITTEE 

of the 

MUNITIONS BOARD 

No. 1593 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison 5, Wisconsin 

In Cooperation with the University of Wisconsin 
- - - - 

1 

ACETYLATED WOOD 

By HAROLD TARKOW and ALFRED J. STAMM, Chemists 

E. C. O. ERICKSON, Engineer 

Forest Products Laboratory, 2 Forest Service 

U. S. Department of Agriculture 

Summary 

Experiments at the Forest Products Laboratory have resulted in a new method 

of treating wood with chemical vapors to improve its dimensional stability 

when exposed to severe andprolonged high humidity or immersed in water Over 

extended periods of time. The method involves treatment with vapors of a 

solution of acetic anhydride and pyridine. The pyridine is used to swell 

the wood and the acetic anhydride to replace the relatively hygroscopic 

hydroxyl groups in the wood with less hygroscopic acetyl groups. The pyridine also catalyzes the acetic anhydride reaction. The acetylation thus 

accomplished leaves the wood in a permanently swollen condition by means 

of the bulking action of the acetyl groups within the cell walls. Acetylation to about a 20 percent acetyl content reduces shrinkage about 70 percent as compared with untreated wood. Limited tests indicate the treatment 

is permanent. Resistance to decay and shipworms is improved, and a few 

tests indicate that strength properties are not impaired and may in some 

cases be improved. 

Introduction 

The hygroscopicity of wood is unquestionably one of the properties limiting its use in aircraft. As a result of this property, it undergoes dimensional. changes when exposed to varying conditions of humidity. Many 

methods have been tested at the Forest Products Laboratory and elsewhere 

to improve the dimensional stability of wood. These methods can be segregated into four general classes: 

1

This is one of a series of progress reports prepared by the Forest Products 

Laboratory relating to the use of wood in aircraft. Results here reported are preliminary and may be revised as additional data become 

available. 

2

Maintained at Madison, Wis., in cooperation with the University of 

Wisconsin. 

Rept. No. 1593 -1- Agriculture-Madison 
--- 

1. Surface coatings or pore sealers to retard the rate of movement of 

moisture into the wood substance. 

2. Introduction of a nonvolatile substance (bulking agents) in the swollen 

wood that keeps it at its approximate swollen dimensions. 

3. Chemical modification of the wood components, such as by heat treatments (11, 13).3 

4. Introduction of occasional cross links (e.g., formaldehyde) between 

swelling units, thereby reducing their tendency to separate. 

The methods of class 1. are successful only when the changes in humidity 

occur over short periods of time, as such coatings as paints, oils, and 

waxes are ineffective when humidity changes extend over long periods of 

time (2, 6, 12). 

Two general methods have been tried that are classifiable under class 2. 

Method (a) utilizes water-soluble, hygroscopic materials such as sugars 

(1, 10) and hygroscopic salts (9) that keep the wood swollen as a result 

of their bulking action in combination with the excess moisture that they 

hold in the wood at relative humidities above those in equilibrium with a 

saturated solution of the treating agent. Method (b) involves the use of 

water-insoluble materials or materials that can be made water-insoluble 

after impregnation (14, 15). Following drying to remove the solvent, and 

heating if the material is heat-polymerizable, the wood is left in a partly 

swollen condition because of the bulking action of the deposited or polymerized material. Since the water-swollen dimensions are not altered by 

the treatment, the treated wood goes through a smaller volume change when 

immersed in water than does a corresponding control. 

Studies are continually being made at the Forest Products Laboratory to 

find new ways of stabilizing wood. One of these studies is concerned with 

the acetylation of wood. 

Chemistry of Acetylation 

Wood is composed essentially of polysaccharides (cellulose and hemicellulose) and lignin in proportions of about 70 and 30 percent, respectively. 

Studies have indicated that the water-attracting property of wood is due 

in large measure to the hydroxyl groups located on the lignin residue and 

on the polysaccharide chains. This finding suggested the possibility of 

stabilizing wood by replacing some of the hydroxyl groups with less hygroscopic groups by a method that would leave unchanged the residues to which 

3Underlined numbers in parentheses refer to Literature Cited at end of 

this report. 

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the groups are attached. It would not be necessary to replace all the 

hydroxyl groups, as x-ray investigations have shown that when water enters 

wood or other cellulosic materials the crystalline portions do not come in 

contact with the moisture. Only those hydroxyl groups that are located on 

the surface of the micelles or in the amorphous regions of wood come in 

contact with water and, presumably, only these need therefore be replaced. 

Many types of reactions are known in which hydroxyl groups participate. 

The most practical of these is the interaction of an hydroxyl group with 

an acid anhydride. This reaction is called esterification. In particular, 

when acetic anhydride is used, the reaction is called acetylation. The 

reaction is as follows: 

where R may represent a lignin residue, a polysaccharide chain, or any 

other organic radical. Unfortunately, the reaction proceeds only in the 

presence of a catalyst. This is usually a strong mineral acid. In addition, if R is a large molecule, a swelling agent (frequently called an 

activating agent) must be used to open up and expose the more intimate 

structure to the acetylating agent. The swelling agent is usually acetic 

acid. Thus, in the commercial manufacture of cellulose acetate, cotton 

linters are first swollen in acetic acid and are then treated with a very 

dilute solution of sulfuric acid in acetic anhydride. The sulfuric acid 

catalyzes the esterification reaction and promotes the partial breakdown 

of the cellulose chains. Its effect upon the cellulose, if kept within 

bounds, is not harmful in making plastics; in fact, it is desirable. 

Wood can be acetylated in a similar manner. Owing to the presence of the 

strong mineral acid, however, degradation occurs and significant losses in 

mechanical properties result. Acids weaker than sulfuric acid are ineffective as catalysts. 

Certain organic bases, such as pyridine, are known to catalyze the reaction 

between the hydroxyl group and acetic anhydride. Hess (5) found that cellulose could be acetylated to the triester stage without any apparent degradation. He replaced the water in water-swollen cellulose with pyridine and 

then treated the pyridine-swollen material with a solution of pyridine in 

acetic anhydride. Mullen and Pacsu (7) have shown that starch can be 

acetylated with pyridine and acetic anhydride without breakdown in structure. Staudinger (16) showed, through viscosity studies, that cellulose 

acetate prepared by Hess' method has the same degree of polymerization as 

that of the original cellulose, 

Pyridine is a tertiary amine with a characteristic nauseating odor. It 

is obtained commercially from coal tar. Certain other amines can be used 

Rept. No. 1593 -3-
instead of pyridine as basic catalysts. One requirement is that the amine 

be a tertiary amine, for mono and secondary amines react with acetic anhydride. 

In base-catalyzed acetylations, acetic acid cannot be used as the swelling 

agent, since the acetic acid would react with the pyridine to form the salt, 

pyridine acetate. Instead, the cellulosic material is generally first 

swollen in water. The water in the swollen material is then replaced with 

pyridine. The material thus treated retains its water-swollen dimension 

and is readily acetylatable with acetic anhydride. 

Preliminary Forest Products Laboratory experiments to learn the effect of 

pyridine on wood showed it to be a powerful swelling agent that swells wood 

more than 25 percent more than does water. Since cellulose does not swell 

appreciably in pyridine, while isolated lignin does swell (and slowly dissolve), it may be concluded that it is the lignin in the wood that responds 

to the pyridine. 

These findings led to tests to ascertain the feasibility of swelling wood 

directly with pyridine rather than preswelling it with water as in Hess' 

acetylation method. Small maple cross sections were immersed in solutions 

of pyridine in acetic anhydride (liquid-phase treatment) at various temperatures for various periods of time. It was found that acetyl contents 

of the order of 20 percent could readily be obtained with antishrink efficiencies of the order of 70 percent. The antishrink efficiency is defined 

as: 

Percent swelling - Percent swelling of 

100 x of control treated material 

Percent swelling of control 

That is to say, the tendency of the maple to swell and shrink in water was 

permanently reduced by 70 percent. Interestingly enough, the acetylated 

specimens had increased ovendry dimensions; Insofar as other visual properties were concerned, however, the treated wood seemed unaltered. 

This liquid-phase process has a serious drawback, Owing to the cellular 

nature of wood, a much larger quantity of acetylating solution is imbibed 

than is actually needed, and the excess must be removed from the treated 

wood. To eliminate this costly and time-consuming factor, attempts were 

made to acetylate wood with vapors from a pyridine-acetic anhydride solution 

(vapor-phase treatment). Under these conditions, only the cell walls absorb 

the vapors. The cell cavities remain virtually empty. 

With the vapor-phase treatment also, sufficient acetylation was obtained to 

reduce the swelling tendency of the wood by 70 percent. This stabilization 

was, moreover, obtained with the use of much less chemical than was required 

in the liquid-phase treatment, For example, spruce immersed in the liquid 

acetylating solution absorbed 227 percent of its weight of solution; when 

Rept. No. 1593 -4-
treated with vapors at the same temperature, the same wood absorbed only 

50 percent of its weight, Yet in both instances the same acetyl content 

(21 percent) was obtained. Acetyl content is expressed as the percentage 

gain in weight over the original ovendry weight, 

The vapor-phase technique requires that the basic catalyst be relatively 

volatile. Some tertiary amines, such as quinoline, cannot be used in 

vapor-phase acetylations for this reason. 

Because of the smaller consumption of chemicals, it was decided to concentrate all further work on the vapor-phase treatment, 

Acetylation Equipment 

Acetic acid is a byproduct of the acetylation process. The treating chamber, 

consequently, must be constructed of material capable of withstanding the 

corrosive action of acetic acid at elevated temperatures. In addition, the 

material should not absorb vapors, for this would increase the consumption 

of chemicals. The ordinary metals and most of the common plastics are 

readily attacked by acetic acid. Rubber swells appreciably in the acetylating solution. 

A number of acid-resistant bricks were examined as possible materials for 

the treating chamber. One of these proved remarkably resistant and absorbed virtually no chemical over long periods of immersion. A commercial 

acid-resistant mortar was also found to be suitable for joining the bricks. 

Stainless steel (Nos. 301 and 302) was found to have fairly good resistance 

to attack. The incorporation of small amounts of molybdenum results in a 

steel (No. 316) that is very resistant to attack by acetic acid. Anew 

commercial plastic, polytetrafluorethylene (8), was examined in the form 

of thin films and found to be inert to boiling acetic anhydride, acetic 

acid, and even to nitric acid. The film can be heat sealed at about 350° 

C. (662° F.). 

The early Forest Products Laboratory experiments were carried out in glass 

bottles and small stainless-steel bombs. Later, a chamber 26 by 22 by 18 

inches in size was constructed of Douglas-fir plywood, with internal coils 

located near the bottom of the chamber; a tray on top of the coils; a tube 

leading from the tray to the outside of the chamber, through which treating solution was introduced; a two-blade fan for circulating vapors; and 

glass rods located at the top of the chamber, from which material to be 

treated could be suspended. After several weeks of operation, the walls 

of the chamber were badly swollen and delaminated. A chamber of similar 

size was then constructed of solid wood and lined with No. 316 stainless 

steel. Most of the data in this report were obtained from veneers treated 

in these two chambers. 

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Treating Variables 

Pressure 

The first experiments were carried out in stoppered glass bottles. Subsequent experiments, in which the bottles were open to the atmosphere 

through reflux condensers, showed that acetylation can be achieved readily 

at atmospheric pressure. The construction of large treating chambers is 

thus simplified . 

Time, Temperature, and Composition 

Time, temperature, and composition of the acetylating solution are factors 

in the process. The time required to obtain a degree of acetylation that 

gives 70 percent antishrink efficiency decreases with increasing temperature. At 90° C. (194° F.), about 6 hours are required to treat 1/16-inch 

hardwood veneers to a 20 percent acetyl content when the liquid phase consists of 20 percent of pyridine in acetic anhydride. A higher concentration of pyridine reduces the treating time only slightly. Softwoods, under 

the same conditions, require about 10 to 12 hours for adequate acetylation 

(25 percent acetyl content). With softwoods, however, increasing the concentration of pyridine to 50 percent in the liquid phase has a pronounced 

effect in reducing the treating time. 

Moisture Content 

If the treatment is carried out in a vapor-tight chamber that completely 

excludes the atmosphere, then the moisture content of the wood is an important factor in the acetylation process. Here, the rate of acetylation 

decreases as the moisture content increases. At atmospheric pressure the 

rate of acetylation does not seem to depend on the moisture content of the 

wood. In any case, the moisture content should be as low as possible because acetic anhydride is used up in reacting with the moisture in the wood 

to form acetic acid. About 2 percent appears to be a reasonable moisture 

content of wood for acetylation. 

Thickness of Wood 

The rate of vapor-phase acetylation is determined by the rate of diffusion 

of the vapors into the wood. The rate of diffusion into a porous solid 

varies inversely as the square of the thickness. Because of this, the 

treatment of thick stock requires a prohibitively long period of time. 

From available data it appears that 1/8-inch-thick veneer is the thickest 

wood that can be treated uniformly in a reasonable length of time. For 

this reason all the data here presented, except those for balsa blocks 

Rept. No. 1593 -6-
1/2 inch thick in the fiber direction, were obtained from veneer 1/8 inch 

or less in thickness. Fundamental studies on diffusion rates in wood (18) 

indicate that diffusion in the fiber direction ranges from 10 to 15 times 

as great as at right angles to the fiber. Sections 1 to 2 inches long in 

the fiber direction should thus treat about as fast as 1/8-inch-thick 

veneer. 

Species 

Species that have been acetylated include yellow birch, sugar maple, sweetgum, yellow-poplar, basswood, mahogany, balsa, Sitka spruce, white spruce, 

and Douglas-fir. With same of these, detailed studies were made of the 

effect of acetylation on the degree of stabilization. The results are 

presented in figure 1. The extent of dimensional stabilization imparted 

by the treatment increases almost in a linear manner with increasing acetyl 

content up to a certain value, beyond which it increases but slightly with 

further acetylation. An antishrink efficiency of 70 to 75 percent seems 

to be the highest stabilization that can be readily obtained. 

Stabilization of this degree is obtained at different acetyl contents for 

hardwoods and softwoods. For hardwoods, such stabilization occurs between 

18 and 20 percent, and for softwoods, it occurs at about a 26 percent acetyl 

content. For any given acetyl content, hardwoods have the greater dimensional stability. For example, at 16 percent acetyl content the antishrink 

efficiency of hardwoods is about 63 percent, that of softwoods about 50 

percent. 

This difference in antishrink efficiency between hardwoods and softwoods is 

not fully understood. Hardwoods in general have higher hemicellulose content than softwoods. In addition, it is generally believed that hardwood 

lignin is more reactive chemically than is softwood lignin. Lignin isolated from softwoods by the Willstätter method can be acetylated to the 

extent of 20 percent, whereas the corresponding lignin from hardwoods (beech) 

can be acetylated to the extent of 38 percent (17). Another possible reason 

for the difference in reactivities between hardwoods and softwoods may be 

the difference in Location of the lignin. Thus, according to Harlow (3, 4), 

although the middle lamella is made up essentially of lignin, a portion of 

the total lignin of softwoods is in the secondary layer of the cell wall. 

It may require a more drastic condition to treat this lignin than the 

lignin in the middle lamella. 

Properties of Acetylated Wood 

Appearance 

Little change in natural color is observed in acetylated spruce, Douglasfir, basswood, and balsa. Birch often is darkened, although some lots of 

Rept. No. 1593 -7-
veneer showed no color change after treatment. The surface of an acetylated sheet of veneer often feels somewhat rougher to the hand than does 

the untreated veneer. If, however, it is pressed at as low a pressure as 

100 pounds per square inch and 150° C. (302° F.), the surface becomes and 

remains extremely smooth. 

Dimensional Stability 

The improved dimensional stability of acetylated wood has already been 

discussed and is illustrated in figure 1. 

Reduction in Hygroscopicity 

Table 1 gives values for the percentage reduction in hygroscopicity and in 

the antishrink efficiency for acetylated spruce between the ovendry condition and the indicated relative humidity. 

The percentage reduction in hygroscopicity is always somewhat less than 

the percentage reduction in swelling (table 1). The data in table 1 show 

that when spruce acetylated to 30 percent acetyl content is oven dried and 

then conditioned, for example to equilibrium with 80° F. and 80 percent 

relative humidity, its gain in weight is 37 percent of the gain in weight 

shown by the control, and that its increase in dimensions is only 24 percent of the increase shown by the control. Such differences between reduction in hygroscopicity and reduction in swelling have been obtained repeatedly with other specimens. They exist regardless of whether the percentage 

reduction in swelling is based on tangential or volumetric measurements. 

Permanence of Stabilization 

Esters of organic campounds are known to be susceptible to hydrolytic 

attack (loss of acetyl groups and regain of hydroxyl groups). Consequently, 

information was sought concerning the permanence of the stabilization 

brought about by acetylation under exposure to moisture. 

Acetylated spruce veneer was subjected to 10 cycles of humidity from 30 

percent through 97 percent relative humidity over a period of 4 months. 

No change in degree of stabilization was observed. 

Acetylated birch was suspended over a saturated solution of sodium chloride 

at 80° C. (176° F.) for 3-1/2 days. The relative humidity of the air above 

the aqueous solution was 75 percent, No change in degree of stabilization 

was observed. 

Acetylated birch was immersed in a 9 percent solution of sulfuric acid in 

water at room temperature for 18 hours. The acid was then washed out and 

Rept. No. 1593 -8-
the degree of swelling of the treated wood from the ovendry to the waterswollen condition was measured, The treatment with acid had no effect on 

the acetyl content and on the extent of stabilization. When the acid 

soaking was carried out at 40° C. (104° F.), a reduction in the degree of 

stabilization was noted, with the antishrink efficiency dropping from 75 to 

65 percent. 

Acetylated birch panels and untreated controls were immersed in the warm 

waters of the Gulf of Mexico off the coast of Florida. The salinity of 

this water ranged from 3 to 5 percent. After 1 year the controls were 

badly damaged by shipworms (Teredo), yet the acetylated panels were not 

affected. Although the experiment was designed to study the resistance of 

acetylated wood to shipworms, the observations made after 1 year gave added 

evidence of the permanence of the stabilization. 

These limited experiments suggest that the stabilization imparted to wood 

by acetylation is permanent and that fairly extreme conditions of weathering have little hydrolytic effect on the acetyl groups. 

Moisture Absorption 

Acetylated wood differs from resin-treated wood in that the communicating 

capillary structure is not blocked by a deposited material. Acetylated 

wood, therefore, takes up liquid water within the coarse capillary structure about as readily as does untreated wood. Less water, however, enters 

the cell walls as a result of their reduced hygroscopicity. Acetylation 

thus does not exclude water, even though it reduces the hygroscopicity 

and dimensional changes of the wood. 

Moisture Diffusion 

Acetylation has little effect on the rate of diffusion of moisture at low 

relative humidities. At high humidities it has an appreciable effect in 

reducing the diffusion. This is due to the ability of acetylation to reduce bound-water diffusion, which is more prevalent at high humidities. 

Swelling of Acetylated Balsa in Organic Fluids 

Acetylated balsa was immersed in several organic fluids. After 7 days the 

extent of imbibition and swelling was determined. The results are given 

in table 2. 

In the alcohols (ethylene glycol and isopropyl alcohol), the acetylated 

specimens swell less than do the untreated specimens, This reduction in 

swelling, however, is not so great a6 that in water. This is not unexpected, for the organic nature of the alcohols should make them more effective as swelling agents for acetylated wood than water. 

Rept. No. 1593 -9-
Specific Gravity 

With increasing acetyl content, the ovendry dimensions of the wood increase 

(fig. 2). The percentage increase in dimensions of acetylated wood, like 

that which occurs during the adsorption of water by untreated wood, is 

greatest for the species with the greatest specific gravity. Because of 

this, for a given acetyl content, a species of high specific gravity undergoes a smaller percentage increase in this property than one of low specific 

gravity (fig. 3). At an acetyl content of 20 percent, for example, the 

ovendry specific gravity of maple is increased by 8 percent, that of spruce 

by 12 percent, and that of balsa by 14 percent. 

On the other hand, when based on the weight and volume under normal conditions, the percentage increases in specific gravity are less than those 

based on ovendry weight and volume. At 65 percent relative humidity, for 

example, untreated spruce has a moisture content of 12 percent, whereas 

spruce with an acetyl content of 20 percent, having an antishrink efficiency 

of 60 percent and a reduction in hygroscopicity of 50 percent, has a moisture content of 6 percent. Under these conditions, the increase in specific 

gravity at test over that of untreated wood is 9 percent rather than 12 percent. Similarly, the specific gravity increase of maple at test (68 percent relative humidity) is only 5 percent in contrast to 8 percent for the 

dry wood, 

Exposure to Ultraviolet Light 

Exposure of untreated wood to ultraviolet light usually results in a darkening of the wood (noticeable even after 1 day of exposure). Under similar 

conditions, acetylated wood either does not change color or is slightly 

bleached. 

Resistance to Decay 

The resistance of acetylated balsa to several wood-destroying organisms 

was investigated. After 3 months of exposure, Poria microsporia, P. 

incrassata, and miscellaneous organisms naturally occurring in soil were 

found to have negligible effects on treated and untreated balsa blocks. 

Poria versicolor had no effect on acetylated balsa, whereas it severely 

attacked the controls, with the loss in weight being as much as 50 percent. 

Acetylated sweetgum and yellow-poplar veneer were exposed to 97 percent 

relative humidity, together with controls. After several months the controls were coated with blue stain, but the treated veneers were clean. 

Acetylated birch panels were inserted in termite-infested soil. After 5 

years, the panels showed no signs of attack, whereas the average life of 

the controls was 2.7 years. 

Rept. No. 1593 -10-
Shipworm damage in heavily infested Florida waters, as already mentioned, 

was absent from acetylated wood after 1 year, although untreated controls 

were badly damaged. 

Paintability 

Panels consisting of Douglas-fir untreated cores faced with 1/32-inch acetylated and nonacetylated veneers were subjected to outdoor southern weather 

exposure with and without various heavy coatings of paint and enamel finishes. After two years the uncoated unacetylated panels showed very marked 

checking and extensive firring of wood fibers. The uncoated acetylated 

panels showed very slight, difficultly discernible checking. Some firring 

of wood fibers also occurred. 

There was not much evidence of improvement in the weatherability of' the 

coatings applied over acetylated wood, as the coatings were heavy and relatively extensible. Thinner nonextensible coatings, such as airplane or 

automobile finishes, might have shown larger differences. 

Coefficient of Linear Thermal 

Expansion of Acetylated Wood 

Table 3 gives the meager data available for thermal expansion of acetylated 

wood. Acetylation increases the coefficient of thermal expansion slightly. 

Strength Properties 

Data pertaining to the strength properties of acetylated wood are as yet 

fragmentary. Only a few studies have been made to determine the degree 

of degradation, if any, that may result in wood on acetylation. These 

studies were confined to an examination of certain strength properties of 

yellow birch, Sitka spruce, balsa, and basswood at a number of acetyl contents. Strength data for matched specimens of treated and untreated woods 

are reported in tables 4 to 8, inclusive. 

Tables 4 and 5 summarize the toughness, flexural, and tensile properties 

of normal and acetylated Sitka spruce, yellow birch, and basswood obtained 

from different lots of 1/16-inch rotary-cut veneer at various acetyl contents. 

Table 6 gives the same properties obtained with 1/8-inch rotary-cut birch 

veneer. 

Table 7 summarizes the tensile, compressive, and shear properties of normal 

and acetylated parallel-laminated wood made from nine plies of 1/16-inch 

rotary-cut Sitka spruce veneer. Shear values parallel to grain were obtained 

by means of the plywood joint test employing 1- by 3-inch kerfed specimens. 

Rept. No. 1593 -11-
Table 8 summarizes the toughness, shear, tensile, and compressive properties of norma1 and acetylated balsa. Shear and compression specimens were 

prepared by treating small balsa blocks (2-inch cubes). Tension specimens 

were made by laminating four sheets of flatsawn balsa 3/16 inch thick following treatment, Toughness specimens 1/2 by 1/2 by 3-1/2 inches in size 

were obtained from treated material, 5/8 inch square by 4 inches in length. 

Compression specimens were loaded over the entire 2- by 2-inch end surface 

at no-load speeds of 0.006 and 0.012 inch per minute for tests parallel 

and perpendicular to the grain, respectively. Shear specimens were standard 2- by 2- by 1-1/2-inch block shear specimens. 

Toughness tests were made in the Forest Products Laboratory intermediatecapacity toughness machine over a 2-1/2-inch span. Tensile specimens were 

tested in self-alining Templin grips (gripped flatwise). 

Elongation data were obtained by means of a separable-type extensometer of 

2-inch gage length, equipped with a 0.0001-inch dial gage. For the spruce 

and balsa laminated specimens (tables 7 and 8), the extensometer was 

attached to the edges of the laminations. These specimens were 1 inch wide 

by nominal laminated thicknesses by 16 inches long, shaped to have a 2-1/2­

inch long central section 1/4 inch wide. The taper followed a 60-inch 

radius on each edge. 

The ends of all balsa tension specimens were reinforced with two inserts of 

1/8-inch birch to prevent end crushing in the grips during test. These 

inserts were glued in 3-inch-deep saw kerfs cut parallel to the balsa grain 

and across the laminations, with the grain of the birch normal to the plane 

of the plies. 

The data presented in the tables represent the averages of test values of 

specimens conditioned at 75° F. and 65 percent relative humidity. Consequently, these data for untreated and acetylated materials represent 

strength values at different moisture contents, since at 65 percent relative humidity the equilibrium moisture content of normal wood is 12 percent, whereas that for treated wood is 6 percent or less. Inasmuch as a 

similar difference in moisture content is to be expected under normal use 

conditions, it seems legitimate to compare the strength values without 

adjustment for the effect of differences in moisture content. On this 

basis, it may be concluded from the data that acetylation has not significantly reduced any strength property; in fact, in some instances acetylation seems to have improved the strength property. 

Weathering of Acetylated Balsa 

Sandwich Constructions 

Sandwich constructions were prepared with treated and untreated balsa cores 

glued to birch and glass-fiber faces. These were exposed to various artificial weathering conditions, following which the tensile strength of the 

Rept. No. 1593 -12-
composites was determined. Unfortunately, the failures occurred in the 

plies of the skins rather than in the core or in the glue joint between 

core and face. 

New sandwich constructions were made using aluminum-block faces primed with 

a vinyl-resin composition and glued to treated and untreated balsa cores 

with a resorcinol glue. The balsa specimens consisted of matched material 

ranging in density from 6 to 8 pounds per cubic foot. The data obtained 

with these constructions are presented in table 9. Column 10 of table 9 

gives the ratios of strength values obtained with treated specimens to 

values obtained with untreated specimens. The ratios are close to unity, 

indicating little difference between the two groups. It had been expected 

that the ratios would be appreciably greater than unity because of the improvement in dimensional stability of the treated wood. Since the failures 

occurred mainly in the glue joints (80 percent glue failure was recorded 

for both groups), it is obvious that the tests do not reflect the true 

effect of acetylation on the tensile strength of these sandwich specimens, 

and thus any improvement in the strength of acetylated balsa resulting 

from reduced swelling and shrinking will only be useful when the glue bond 

between metal and wood can be improved. 

Conclusions 

The information given in this report on the pyridine-catalyzed esterification of wood indicates that appreciable dimensional stabilization and improvement in surface properties of the wood can be obtained without impairing the mechanical properties of the wood. Decay and shipworm resistance 

are improved, and the treatment appears permanent under rigorous conditions 

of exposure. Existing knowledge is not sufficient, however, to complete 

the evaluation of acetylated wood. Among factors affecting its use as yet 

unknown or insufficiently established are its gluability, its paintability, 

the possibility that treated wood will corrode metal, and the economic 

aspects of the process. 

Addendum 

Mechanism of the Stabilization 

Imparted by Acetylation 

The data presented in table 10 are helpful in arriving at an understanding 

of the mechanism of the stabilization imparted by acetylation. 

The data of table 10 reveal that acetylated wood has the same water-swollen 

volume as does the untreated wood, but that its ovendry volume is greater. 

Consequently, on immersion in water, the untreated specimen undergoes a 

Rept, No. 1593 -13-
-- 

-- 

greater change in volume than does the treated specimen. From this it 

appears that acetylation improves the dimensional stability of wood by 

virtue of the fact that a nonleachable bulking agent, the acetyl group, 

is laid down in the cell wall. 

It may be argued that the replacement of the hydroxyl groups by the acetyl 

groups causes the stabilization as a result of the replacement of hydrophilic groups by less hydrophilic groups. Evidence, however, has been 

accumulated that suggests that the reduced polarity is not as probable 

a cause of stabilization as the bulking effect. Wood was acetylated, propionylated, and butyrylated. If stabilization resulted from the replacement of hydroxyl groups by the acyl group (acetyl, propionyl, or butyryl) 

for a given acyl content, the acetylated wood should have the greatest antishrink efficiency, the butyrylated wood should have the lowest, and that of 

the propionylated wood should lie between that of the acetylated wood and 

the butyrylated wood. This would be so because of the increasing molecular 

size and, therefore, because of the decreasing number of hydroxyl groups 

replaced as the molecular size of the acyl group increases. 

Actually, all three treated specimens have the same antishrink efficiency. 

Yet only two-thirds of the hydroxyl groups replaced in acetylated wood are 

replaced in butyrylated wood. Thus it seems that the extent of stabilization obtained by acylating wood depends primarily on the bulk of the acyl 

groups laid down per unit weight of wood. The single curve of figure 4, 

in which the volume of the acyl groups (acetyl, propionyl, or butyryl) per 

unit weight of wood is plotted against the antishrink efficiency, holds for 

acetylated, propionylated, and butyrylated wood, The calculated specific 

volume of the acetyl group, the propionyl group, and the butyryl group is 

0.902, 1.00, and 1.055 cubic centimeters per gram, respectively. 

Stabilization by acetylation thus belongs in method (b) of class 2 of the 

classes of stabilization listed in the introduction. The bulking effect 

that is, the increase in ovendry volume on acetylation is generally 

equal to the volume of the acetyl groups. This is revealed in figure 5, 

in which the volume of the acetyl groups per gram of wood is plotted 

against the geometrical swelling of the wood. The volumes of the acetyl 

groups are obtained by multiplying their weights by their calculated 

specific volumes. The slope of the curves for spruce and maple is unity. 

This, of course, means that the acetyl groups add their volumes to that of 

the wood substance. The balsa curve is interesting. For every volume of 

acetyl groups taken on by this wood, the wood swells by two volumes. These 

relations are similar to those found for wood and water. Thus, in the 

maple-water or spruce-water systems the water adds its volume to that Of 

the spruce or maple, whereas in the balsa-water system for every volume 

of water taken up by the balsa the wood swells by about two volumes. 

Rept. No. 1593 -14- .4-30 
Literature Cited 

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Browne, F. L. Ind. Eng. Chem. 25:835 (1933). 

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Hunt, G. M. U. S. Dept, Agric. Circ. No. 128 (1930). 

Mullen, J. W., and Pacsu, E. Jour. Amer. Chem. Soc. 63:1487 (1941). 

Plunkett, R. J. U. S. Patent 2,230,654, February 1941. 

Stam, A. J. Jour. Amer. Chem. Soc. 56:1195 (1934). 

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Stamm, A. J., and Seborg, R. M. Trans. Amer. Inst. Chem. Eng. 

37: 385 (1941). 

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Wise, L. E. Wood Chemistry, Monograph 97, p. 532 (1944). 

Reinhold Publishing Corp., New York. 

Rept. No. 1593 -15-
Table 1. --Percentage reduction in hygroscopicity and 

antishrink efficiency of' acetylated Sitka 

spruce (30 percent acetyl content,) between 

the ovendry condition and various relative 

humidities 

Table 2.--Swelling of normal and acetylated balsa 

in organic fluids and water 

Rept. No. 1593 
Table 3.--Coefficients of linear thermal expansion 

of acetylated Sitka spruce 

Rept. No. 1593 
Rept. No. 1593 
Table 5.--Tensile and toughness properties of matched untreated and acetylated Sitka 

spruce, yellow birch, and basswood rotary-cut veneer 1/16 inch thick 
Table 6. --Some properties of matched untreated and acetylated 

yellow birch rotary-cut veneer 1/8 inch thick 

Rept. No. 1593 
Table 7.--Some properties of parallel-laminated matched untreated and acetylated wood made from 9 plies 

of 1/16-inch rotary-cut Sitka spruce veneer1 

Rept. No. 1593 
Table 8. --Some properties of matched untreated and 

acetylated balsa wood 

Rept. No. 1593 
Z M 67246 F 
Table 10.--Volumetric changes in the system, acetylated 

spruce-water 

Rept. No. 1593 
Figure 1.--Relationship between the acetyl content and the antishrink 

efficiency for several species of wood. 

Z M 67247 F 
Figure 2.--Relationship between the acetyl content and the percentage 

increase in oven-dry volume. 

Z M 67248 F 
Figure 3.--Relationship between the acetyl content and the percentage 

increase in specific gravity. Z M 67249 F 
Figure 4.--Relationship between volume of acyl groups per gram of wood 

and the antishrink efficiency. 

Z M 67250 F 
Figure 5.--Relationship between the volume of the acetyl groups and the 

external volumetric swelling. 

Z M 67251 F