PIES I N-TRATE 1U1AINATED
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University ol Wisconsin
Digitized by the
RE.SIN-TREATED, IxiI:TATED, COMPRESSED WOOD-I
By A. J. STA'i.,, Principoal Chemist
and R. M. SEBORG, Assistant Chemist
The authors have shown in previous -publications (3, 41, 5) that the
formation of synthetic resins within the intimate cell-wall structure of wood
from polar resin-forming constituents greatly increases the moisture resist-
ance of the wood. Moisture adsorption and swelling and shrinking have .been
reduced to one-fourth of normal under true equilibrium conditions and to a
considerably greater extent under normal use conditions. Plywood made entire-
ly from the treated plies or made with only the face plies treated showed a
marked decrease in face checking under weathering conditions and a marked de-
crease in the passage of moisture through the plies under a relative humidity
gradient (4, 5). The decay resistance of the wood was appreciably increased
(4, 5) and the compressive strength properties were increased in greater
proportion than the increase in weight of the wood (3).
A number of different resin-forming materials have been tried. The
most successful of these is an alkaline catalyzed, practically unpolymerized,
phenol-formaldehyde resin-forming mix with a pH of about 8 that is soluble
in water in all proportions. The urea-formaldehyde resin-forming Wstcms
tried were considerably less effective than the phenol-formaldehyde systems
in permanently reducing shrinking and swelling. This is perhaps partially
due to the fact that the systems were initially too far polymerized for the
resin-forming mixes to diffuse appreciably into the fine cell-wall structure.
Vinyl, styrine, Glyptol, and methyl methacrylate resins were practically in-
effective in permanently reducing the hygroscopicity of wood. This appears
to be due to the fact that the monomers of these resins do not have suffi-
cient affinity for wood and do not enter and bond to the cell-wall structure.
The extent to which wood swells in a resin-forming solution beyond
the swelling in the solvent alone is a good gauge of the affinity of the wood
for the resin-forming constituents, Aqueous phenol-formaldehyde resin-form-
ing solutions cause considerably more swelling of wood beyond the swelling
in water than is caused by any of the other resin-forming systems tried.
The only resin-forming systems tried that are not so effective in reduci..-
the shrinking and swelling of' the wood subsequent to the curing of the resin
as would be expected on the basis of the swelling of the wood in the original
resin-forming mix are very high alkalinity commercial phenol-formaldehyde
resin-forming mixes. The high alkali concentrations are used to keep
appreciably prepolymerized resin in solution, On treating wood with such
presented before the'American Institute of Chemical Ei.:i.eers, Chicago, Ill.,
May 21, 1941. (To be published in Institute's journal.)
solutions, the wood swells because of the selective adsorption of alkali and
not because the resin forming constituents are entering the cell-wall structure.
Bakelite Resinoid XR5995, a commercial phenol-formaldehyde resin-form-
irn mix that is miscible in water in all proportions, was used as the treat-
ing mat,'i;1 in all the experiments. It has better keeping qualities than
any of the mixes that have so far been made at the Forest Products Laboratory.
The veneer can be treated by a diffusion of the resin-forming mix into
green veneer direct from the cutter knives by merely soaking it in an aqueous
solution of the mix (4, ). The time required for the veneer to take up the
desired amount of resin-forming constituents will vary directly as the square
of the thickness of the veneer, directly as the specific gravity of the wood,
and inversely with the moisture content of the wood and the temperature.
Green swootgum veneer 1/32 inch thick took up about 40 percent of its dry
weight of resin-forming constituents from a 50 percent aqueous solution of
Bakelite Resinoid XR5995 by diffusion in 1 hour at 100 F.
The veneer can also be treated in the dry condition by the cylinder
treating method. The veneer is placed on edge in a galvanized iron tank in-
side of the treating cylinder. The tank is filled with the treating solution
and the veneer weighted down so that it will remain immersed. The cylinder
is closed and a pressure of 30 to 60 pounds per square inch of air pressure
is applied for 1/4 to 1 hour, depending on the resistance to penetration of
the wood. In the earlier experiments a vacuum was pulled before applying the
pressure (4, 5), but this was found to be unnecessary for most of the veneers
treated in thicknesses up to 1/S inch. It was found that a take-up of solu-
tion equal to the dry weight of the wood was desirable. The concentration
of the nix in water was adjusted so that the final resin content of the dry
wood would be from 30 to 40 percent. This could, in general, be obtained
when the mix consisted of one-half to two-thirds water.
:. the veneer is treated b,' the cylinder method the solution is
carried only into the coarse capillary structure. It takes time for it to
diffuse into the cell-wall structure where it is desired. It is, therefore,
necessary to stack the treated veneer under nondrying conditions for 1 to 2
days with a ca.r.ins thrown over it to cut down circulation.
The veneer is then slowly dried so that the resin-fornin,- constituents
can diffuse from the fiber cavities into the cell walls as water is removed
from the fiber cavities. Dr;'ing on the drying chain of a continuous drier
under nor:ial veneer drying conditions is too rapid. The veneer can be dried
most satisfactorily by putting the sheets on err-e in a drying rack with l/8-
inch spacers between and placing these racks in a dry kiln. The sheets
should be inserted between th s-oacers with the spacers at right angles to
the fiber direction of the sheets. Brass rods or resin-coated steel rods
make ;-- -,d spacers. The dry-.in.:- canr. be done at a teper.-iturc of 150 to 160o
F. or perhaps even a little higher without prcnature setting of the resin.
The relative humidity should be maintained at about 65 to 70 percent to make
possible the continued diffusion of the resin-forming constituents. The
conditions will give an equilibrium moisture content of 8 to 9 percent. This
moisture content is attained in 3 to 4 hours under these conditions with the
Compression and Assembly
Wood treated and dried according to these methods is much more -plastic
at the normal hot-pressing temperatures of 300 to 350 F. than untreated
wood. For example, spruce, cottonwood, or aspen.can be compressed to one-
half their original thickness under as small a pressure as 250 pounds -oer
square inch. This pressure causes only a slight compression of the dry
There is an insufficient amount of resin-forming constituents on the
surface of the dry resin-treated wood to give a bond between plies that are
not compressed. When the wood is highly compressed, enough resin-forming
constituents exude from the plies to give a bond without using additional
bonding material. When the plies are all parallel a good bond is obtained
without the use of additional bonding material, even when the compression is
only about half of the total possible amount. Under conditions where in-
sufficient resin-forming materials exude from the structure to form a good
bond, Tego film has been used for the bonding.
Resin-treated compressed wood has been made up from the dry treated
plies under pressures of 250 to 1,200 pounds per square inch at temperatures
of 300 to 320 F., using pressing periods of 15 to 30 minutes per inch of
original thickness of the wood. It is, in general, desirable to cool the
wood below the boiling point of water before removing it from the press
in order to avoid possible rippling or crazing of the surface. The cooling
in the press, however, is unnecessary in the case of such soft, even-textured
hardwoods as cottonwood or' asp-oen.
The pressing time varies almost directly with the thickness rather
than as the square of the thickness, as might be expected. This is due to
the heat generated within the wood from the resin-forming reaction, contri-
buting to the energy necessary to initiate the reaction a little farther
towards the center of the wood. A thick, compressed panel 2.5 inches thick
was made from a 6.5-inch thick pile of treated plies with thermocouples in-
serted at various places between the plies. In 3 hours the temperature at
the center became equal to the platen temperature, and in another hour
reached a temperature of S0 F. above that of the platens. In making thick
material it is thus desirable to drop the platen tejr.er'ture before the
center of the wood reaches the platen temperature to avoid overcurin: or
Under a pressure of 250 pounds per square inch, spruce, cottonwood,
and aspen are compressed to less than half their original thickness and the
product has a specific gravity of at least 1. The same woods, under a
pressure of 1,000 pounds per square inch, compress to one-third of their
original thickness and give products with specific gravities ranging from
1.3 to 1.4. This corresponds closely to complete compression as the density
of wood substance is 1.46 (2). Different species of poplar and gum compress
but slightly less under the higher pressure.
Compressed wood made in this way is far more homogeneous and water
resistant than the commercial compressed woods now on the market. The resin
within the cell-wall structure and bonding the plies together forms a con-
tinuous homogeneous structure. The resin is formed in the intimate cell-wall
structure and chemically bonded to the hydroxyl groups of cellulose and
lignin that normally take up water rather than being mechanically deposited
in the coarse capillary structure. Figure 1 shl-ws the difference between
the swelling of small blocks of a commercial resin-treated compressed wood and
resin-treated compressed wood made according to the Forest Products Labora-
tory method. The former swelled more in thickness than the theoretical
value for wood of the same specific gravity (about 20 percentt. This is due
to the fact that a considerable part of the dimension change of the commer-
cial product was a relieving of compression rather than a true swelling.
The commercial product was badly checked as a result of the swelling, whereas
the Forest Products Laboratory product retained its smooth, glossy appearance.
The small swelling of the Forest Products Laboratory product occurs
very slowly. This is shown by the percentage weight increase, after differ-
ent times of immersion in water, of specimens (10 by 10 by 1 cm.) of com-
pressed spruce containing 40 percent of resin that were pressed at 1,000
pounds per square inch for 25 minutes at 310 F. The weight increases after
1, 4, and 7 days were 0.5, 1.2, and 1.9 percent, respectively. The German
specifications (1) allow a weight increase for laminated, resin-treated,
comnprcsed wood specimens of the same dimensions after the same periods of
inmersion of 5.0, 7.0, and 8.0 -oercent, respectively.
T'u Forest Products Laboratory compressed wood has a very hard, smooth,
weather-resistant surface. Surface hardness values obtained with a Sword
hardness tester varied from 65 to 90 for different species and anount of
resin present, as compared to 100 for plate glass. Ordinarily smooth Spruce
gave a value of 6. The latter, with a good coat of varnish, gave a value of l1
Mecha-ical tests have not been carried sufficiently far so as to conmare
critically the resin-treated, laminated, compressed wood nade by the Forest
Products Laboratory method with that male according to present con -ercial
practice. Indications are, however, that the Forest Products Laboratory
material will have practically equal, if not better nechaiical properties,
as preliminary strength values for compressed wood made from the mechanically
inferior species are about the sane as those reported for the commercial
product niade from.i naple and beech when compressed to the sane specific gravity.
The data indicate, however, that variations in the mechanical p-oroperties of
compressed wood of the same specific gravity made from different species will
be considerably less than the variations in mechanical properties of differ-
ent species of normal woods.
A few samples of parallel laminated, resin-treated compressed spruce
with a specific gravity of 1.3 gave maximum tensile strengths parallel to
the grain of over 40,000 pounds per square inch, modulus of rupture in
static bending of over 40,000 pounds per square inch, maximum crushing
strengths with the compression parallel to the grain of over 20,000 pounds
per square inch, and modulus of elasticity values from the previous three
properties ranging from 4 to 5 million.
Combination of Compressed and Uncompressed Wood
It would be desirable to be able to produce a plywood with the hard,
dense, water-resistant surface and with the improved mechanical properties
that have just been described without a greatly increased weight and cost
above that of ordinary plywood. This has been accomplished by compressing
the resin-treated face' plies and assembling them with an uncompressed core
in a single operation. Such an assembly is made possible by the plasticizing
action of the resin-forming constituents on the treated plies. Under a
pressure of 250 pounds per square inch the face plies of a number of species
can be compressed to half of their original thickness, whereas a dry un-
treated core of the same wood or a more compression-resistant wood will be
compressed only about 5 to 10 percent. If cost is not a serious item and a
high water resistance of the core, as well as of the faces, is desired, all
the plies may be treated and the core plies procured by heating in an oven
without an applied pressure. In this way the compressive strength of the
core is increased by about 50 percent (1) and the core is even less subject
to compression on assembly than are untreated cores. When assembling a
treated uncured ply with an untreated ply or a treated ply in which the
resin has been procured, it is desirable, but not always necessary to use an
additional bonding material. If the resin-forming constituents are absorbed
by the untreated wood as rapidly as they exude from the treated ply that is
being compressed, a starved joint may result.
The hard, glossy surfaces of the compressed wood are difficult to
glue to each other or to ordinary wood because of the nature of the surfaces.
If it is desired to glue.one or both faces of compressed wood, it is ad-
vantageous to make it up with treated, but procured faces. The treated wood
that has not been compressed can be readily glued with any of the con=.ercial
glues (4., 5).
Because of the difficulty of gluing resin-treated compressed wood,
the manufacture of a combination of compressed wood and uncompressed wood in
two step-os would result in a weaker or less water-resistant bond than is
obtained in the single compression and assembly method which is here
Specimens of plywood with 1/16-inch resin-treated compressed faces
and an untreated 3/S-inch, 3-ply core were soaked in water for several days.
The untreated cores5,took up water and swelled in thickness rather than
transversely as a result of the lateral restraint due to the cross banding.
After air drying, followed by oven drying, the specimens showed no face check-
ing due to the stresses. Similar specimens were put through a temperature
change cycle of 24 hours at 23 F. and 24 hours at 150 F. for over a month.
No visible degrade occurred. Specimens exposed to the weather for a period
of 6 months have shown no face checking in contrast to an appreciable face
checking of the controls with untreated uncompressed faces.
If the resin-treated compressed wood is scratched or marred in any
way, the scratch can be sanded out and the specimen buffed, thus restoring
the original finish.
Dyes have been added to the resin-forming mix. A few dyes, chiefly
of the vat-dye type, uniformly penetrated the structure of woods like cotton-
wood and aspen. All the dyes faded to some extent on light exposure. A few,
however, show promise for out-of-door use and more of them show promise for
Enamel paints used to paint insignia on airplanes g:.ve a smooth 1-
coat finish on the resin-treated compressed faces of spruce, whereas a single
coat on the untreated wood showed an obvious need for building up of the fin-
ish. Weather exposure tests have not been sufficiently long to indicate
the life of the 1-coat job on the resin-treated compressed faces. After 6
months' exposure, however, they look very good in contrast to the faces of
the untreated, uncompressed controls which checked through the finish.
The resin-treated compressed woods described in this paper show
several possible uses in airplane construction. The most promising of these
is the use of the material with the compressed faces on an uncompressed core
for fuselage and wing covering. Because of the increased plasticity of the
treated plies, they should respond nicely to the various types of bag mold-
ing now being tried. It seems hopeful that if the molding is carried on at
pressures sCrnewhat in excess of those now used that the highly desirable
surface finish, water resistance, and improved mechanical properties can
be imparted to the surface without appreciably increasing the weight of the
The highly compressed wood with resin-treated but uncompressed faces
shows promise for use as spar plates which can be readily glued to the ends
of the spars to take the bearing stresses where they fasten on the fuselage.
The treated veneer could also be used to advanta e in mnCiing propeller
blanks varying in specific gravity from one end to the other, or even in
molding the propellers to their final shape. Figure 2 shows one of a number
of possible ways of laying up the treated veneer to obtain blanks with vary-
ing specific gravity from one end to the other. Figure 3 shows the fini-.hel
product. Three wedge-shaped piles of spruce veneer were placed in the press
side by side with the high end of the middle pile at the opposite end from
the high ends of the others and pressed simultaneously so as to distribute
the stresses over the press platens. The highly compressed end of the speci-
mens was subjected to a pressure of over 1,000 pounds per square inch,
whereas the other end was subjected to a pressure just great enough for
assembly, giving a specific gravity of about 1.35 at one end and o.48 at the
Laminated wooden propellers are now being made (Schwarz type) with
compressed hubs by scarfing half inch thickniesses of resin-treated compressed
wood to normal spruce. The compressed wood, however, does not have so
high a water resistance as is desirable. Moreover, the process entails the
gluing of compressed wood to compressed wood and coM.--ressed wood to normal
spruce with cold-press glues so that the finished product does not have the
water resistance nor homogeneity of the Forest Products Laboratory product.
There is a distinct possibility that if the plies are pretailored,
- propellers can be molded in a suitable mold to the finished dimensions,
thus giving a highly finished water-resistant finish to the whole blade.
It is also possible to vary the' specific gravity of the propeller blade at
right angles to the blade direction, as well as in the blade direction. The
tip of the propeller could be uncompreased with a compressed sheathing around
it. This could be accomplished by treating only the outer plies near the tip.
Several other types of uses for the combination of compressed resin-
treated faces on an uncompressed core present themselves. This material
should be highly satisfactory for flooring. The hard, finished surface
should be very resistant to marring and r':ir raising, while the uncompressed
core should furnish the desired resilience. The u-i'-:p cost should be
negligible as no finish is necessary. To maintain stress balance when used
in thicknesses less than 3//4 inch, a bottom treated but uncompressed ply
should be used. Th- uncompressed but treated ply will nail or glue readily.
The material can thus be edge nailed or glued to the subflooring. The sur-
face polish may prove excessive for home use. This can be avoided by
partially curing the face plies at the time of dryir-:.-
The plywood with compressed faces could be used to advantage in furndi-
ture manufacture. It is of interest to note here that the resin-treated
compressed faces are highly alcohol resistant as well as -rater resistant.
The possibility of refinishing by merely sanding and buffing is also of con-
The resin-treated compressed faces serve as better moisture barriers
than do the uncormpressed resin-treated materials (4, 5). The cl. '--r.sed
material should thus be of considerable value for interior paneling.
The treatment of veneer with an unpolymerized water-soluble phenol-
formaldehyde resin-forming mix in such a way that the resin-forming con-
stituents are deposited within the fine cell-wall structure and chemically
bonded to the structure making possible the compression of the treated plies
under much lower pressures than would otherwise be necessary and gives the
product a water resistance not obtainable by other proceeses. The treatment
also m.lkcs it possible to manufacture plywood with compressed resin-treated
faces and an uncompressed core in a single compression and assembly opera-
tion. Properties of the materials are given and possible uses in airplane
construction, flooring, paneling, and furniture manufacture are discussed.
(1) Armbruster, F. Kunststoffe 30:58-62 (194o0).
(2) Sta-in'i, A. J., and Hansen, L. A. J. Phys. Chem. 4l:1007-10l6 (1937).
(3) Stamm, A. J., and Seborg, R. M. Ind. Eng. Chem. 28:ll64-ll69 (1936).
(4) Stamm, A. J., and Seborg, R. M. South. Lbrman. 157-162 (Dec. 15, 1938).
(5) Stamm, A. J., and Seborg, R. M. Ind. Eng. Chem. 31:897-902 (1939).
Figure 1.--Swelling of resin-treated, laminated compressed wood.
From left to right:
1. Commercial material, water soaked for
50 days (54 percent swelling).
2. Commercial material, air dry.
3. Forest Products Laboratory material, water
soaked for 50 days (3.6 percent swelling).
4. Forest Products Laboratory material,
Figure 2.--One means of stacking resin-treated veneer for compressing
to form material with a varying specific gravity from
one end to the other.
Figure 3.--Specimen of resin-treated, laminated, compressed wood
with varying specific gravity from one end to the
other, pressed from material that was stacked as
shown in Figure 2.
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UNIVERSITY OF FLORIDA
3 1262 08927 3352