rOCEST PUOUUCTS LAICIPAT(IUY
IvRsvid August 1943
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconia
:'., -3 T HODUC TS LABC-RATORY UREA PLAS TICIZED 2 ,,OOD
(uL- 1 ,OY)
Discovery of the wood plasticizing properties of urea, a low-cost chemical
now produced synthetically, has opened a new field of research at the
Forest Products Laboratory. DT o of the earliest lines of experimentation
followed in developing the possibilities of the new treatment are described
in this mimeograph. Simple impr nation of wood with urea, followed by
dryin, and bending with the aid of heat;, comprises one branch of investi-
ration. For the sake of convenience, the Laboratory designated the
product made by this process as Uralloy A. The second line of research
involves a thermosetting' process and, by overcoming certain limitations
of the simple urea treatment, has resulted in a markedly different
produAct, designated Uralloy C.
It should be emphasized that in neither phase has development proceeded
beyond the laboratory stage of experimentation. ho extensive commercial
applications have been nade, and it is recoinmended that such applications
be undertaken only after careful study of the properties of Uralloy and
instructions for processinL the material as set forth on the following
paes. However, patents have been granted for both processes. Licenses
for t-,iir free use in the United States may be granted by the Secret r"
of Agriculture to persons or companies who are prepared to make use of
,he process commercially. Patent -o. 2298,7 relates to Uralloy A and
Patent eo. 2,313,953 relates to Uralloy C.
URLA--1. FGJATD THERMOL PLASTIC V'OOD
The ori inal disclosure that wood becomes thermoplastic upon so.l15:J. with
a solution of urea came as a result of experiments in chemical seasoning
of wood ot the Forest Products Laboratory. It was found that re.n 1-inch
oak squares so soaked, and subsequently dried, became plastic and easily
bendabil with the hands upon application of heat. With cooling,, the bent
wood became rigid again, retaining its new shasc.
Essentially, th2 simple urea treatment involves nothing more than this
procedure. A technique worked out for its application on a laboratory .
sceic can be adapted easily to other conditions, such as the home workshop
or vmall woodworking pl nt.
Green wood is required for treatment. An aqueous solution of urea is
formed, usjng equal amounts I wOe iht of water and the chemical. Ti
oo' in pla ced in this solution for a length of time that varies with the
scies being) plas icized. For o t!, the soakini period may be generally
Llimoo f 31P-77~y (rev.)sed)
stated as 10 days for each inch of thickness -- the tiny neaee for
absorption of an amount of urea equal to L) percent of the oven-dr : i
of the wood.
After this treatment, the wood should be dried to a moisture contA o.
to 12 percent. The dryinS time needed likewise varies with the drying
temperature used, wood species, and similar considerations. In no cse,
however, should the tm-.;-rature be higher than 140y F., and for oak it
should not exceed the optimum kiln temperature of 115C F. during< the
initial s, es of 2r.-ing. Prolonged drying at temperatures higher than
l40 Fd. a.ill cause changes of the urea in the wood and thus destroy the
plastic zing effect, /.-i_ drying time for urea-soaked wood crn bhe short r
than thadt of untreated wood going through .he same moisture content cng
for example, if the kiln-drying period of 5-inch Sitka spruce I;' 3 days,
this time can be reduced to iG days with urea-impregnated woode This
accelerated dryl:- rate in the kiln is brought about bhy more severe drying
conditions permitted .... the treatment and should largely offset the tim"
consumed ly the soakingi process.
rh.-n properly dried, the wood is ready for heating and bend .. A tem r '-
ature range of from 212 to 220? I'. is the optimum for bending oper- ton0
T~ii wood can be twisted to the desired shape easilyr by h' r.. Fora Larger
pieces, mechanical aids to bending arn necessary. Properly desi ned
equipment v:ill give much sharper bends than can be achieved w. ith or I .ay
be:.dWI.. methods and with far less breakage to stock.o Bert bedain" re-
sults will be obtained if, after drying, temperature of the wood i<
raised to the optimum temperature for bending, by boiling in the regular
urea solution for a period of 15 or 20 minutes per inch of thic.n.ss.
Good bending can, however, be accomplished by heating the dry treated
wood to the desired temperature in dry air. Upon cooling the wood, .s5
stated earlier, will retain its new shape, provided it is dried to a
sufficiently low moisture content before bending In some cases, a.s in
thick stock, it is desirable to predry to a relatively high moisture con-
tent of 25 to 33 percent, In this ev.e.t the bent produces will hive to
be dried on the form somewhat after bendinr in order to r es rve the
Urea plasticization can also be applied to sawdust for the 0oormation of
solid sheets or panels. It is nce..sary, however, to use; or: dera'le
pressure in the moldirn operation. iFor this, ma.chinery is neededo
Th.: compression technique employed by the Laboratory for molding ur'--
treated sawdust consisted of the fiollowinpv sieps:
"See food Bending (Oct. 1.; ) wi th Aj ppndiz (1 ) on Apaatuforn
in; Bolat Ribs," by T. R. C. ..ilson. FPL Y'oo R "66
D Det :rmine oven-dry weight of sawdust.
2. Add dry urea -- 25 percent of the oven-dry weight of the sawdust.
Add e .,:.h water to make the mass a .te.
.' the sawdust mix to a moisture content of 2 to 3 recent.
I- A 1a suitable lubricant, such as 1 to 5 percent zinc stearate, to the
o. Ilace mix in mold.
7, :xert pressure of 1,0C0 to 1,500 Ibs. per sq. in.
1i. Cool uuder pressure by running water through press platens.
, hen, cool,_, to l40 F., remove product from press,
Th technoi'.ue used here is not necessarily the optimum for all conditions
o .f pre in n or the only vway :prssin can be done. Products manufactured
at the La)oratory by following. these steps were, however, of good str nth
Properties of' the Products
romn limited tests it seems th.t lumber or dimension stock impr'egnated .y
th, sim-le- uArea treatment has stren,-th properties at room temperature
biout equal to those of the normal untreated wood. In color it is
oh ract ristiocally "' or black. It can be handled and used about as
ordinary :ood is used.
Koldin of' th urea--i:.:-egnated saw'-dust in a hot press results in consider-
dlI ..... n oac fl tbecomes ,nte
ly chaed prop ties. T1he color is black or gr ay. ? lateral b e comes
so hcrd thlat it is often difficult to nail. Sawing and oi'.. pro er-
tf ;s are ood. Vrner facings can be applied, but somr, ork will be re-
ure to perfect suitailn 1us. ?. nat:.rial is noncorrosive, so far as
e fastenigs are concerned. Ls s thus far made indicate that
str et. quliti s of the tr'Ated, compressed sawdust are comparable to
o(. tr corn: on p1 .stics.
Thmold;d :o eril is not as resstisnt to v.*ter as is ordinary untreated
wood. It "ill w thstand soaking for a comparatively fw hours before
los ng str rjth through dsinte gr, tion. Its water resistant properties
s'-*, erJtly im rovAd 1y the a, edition of -' Ich chemicals as Afurfral,
rtrbon t r',chlor d or thiourea.
T'/:o 0r'ort.in d oficie'cies afoci; the usfulness of Uralloy A, however,
'P; : r it be of soli worod or sawds t. One of th, mi has alre.'' beh a
.0. U ~r is'
mentioned with relation to compressed sawdust -- its poor water resistance.
Impregnation wit h urea, in fact, actually increases the hygrosco-icity of
wood unless the soaking solution be modified with chemicals which incr ase
water resistance. The other deficiency involves the return ef the r,1', rial
to a plastic condition when reheated to 212 F. Softened thus, it t-.;ds
to lose shape in response to ar.y pressures or stresses it tv b urdr
Th.se two deficiencies naturally place limitations upon the practical
usefulness to which the simple urea treatment can be turned. Those; wiho
devwlo-.'d it are therefore inclined to regard it as more important from
a scientific than from a commercial standpoint. The discovery that urea
plasticizes wood is an interesting development in the study of basic wVoodC
properties. ride commercial applications are not expected because of th.
thermoplastic nature of the complex and because urea tends to darkVn wood
and increases its hygroscopicity. Conceivably, however, th,, !process may
have some important uses where these properties are not objectionable.
UREA-FOR? .LL_,;,. i-, .CS-TTV'3, OOD
The twin problems of preventing resoftening of bent urea-treated wood and
improving its water resistance were in great degree solved by addition of
other chemicals, notably formaldehyde, to the soaking solution. It was
found that this substance, unitin' chemically with urea within the wood
cell walls, formed a synthetic resin which gave the wood considerable re-
sistance to water. Swelling and shrinking were thereby mat--rially reduced,
As soon as the resin was set by heat (polymerized), the wood assumed a
stiffness which could not be noticeably affected by subsequent heating, -t
least at 300 F. It thus became permanently thermoset. Po o'btin nood
diffusion into the wood before th, resin-forming action of the formalde-
1..', and urea takes place, this chemical reaction must be retarded by
buffers -hich maintain the solution in the slightly alkaline range.
A buffered urea-aldehyde solution developed at the La'oratory accomplished
these results. Either of the following formulas ca n be used to m-ke this
solution. The ":r.-redients should be added in the order shown with the mix-
ture heated to not higher than 140 F. until all the urea is dissolv.
7i.- mixture should be cooled to room temperature before adding formaldehyd' .
Formula A Formula E
ter ... ..... ...... 231 parts Xater .. .. . parts
Sodium hydroxide.... 6 Sodium hydroxide.... 6 "
Borax .. 39 "r x.. . . . . "
Ur 380 Dimothylol-uroa ..... "
Formald ehyde (37 Urea ................ 253 "
percenIt solution).. _!!34 "_t
All proportions in these formulas are by wright.
yimeo. "o. R1277 (revised)
To secur. best results, the solution formula must be modified and adjusted
to suit various species c 'e. s the above formulas, which have a pH of
about 10, are admirably adapted to plasticizin, oak, a l,.hly acid wood.
For other woods it may be desirable to adjust the pH nearer to S. This
may b done by omitti:'. the sodium h.-r.-oxide in the formulas, or by ndLirg
10 pFarts of glacial acetic acid, or Ty other suitable means. .'hen the
solution is usId to r-sinify wood in applications ,/;hch do not involve
b -nin-, the proportion of formalde'o.':]Ln can be increased so as to obtain
g reaer water resistant. and increased strength properties.
soaking period necessary to get com,-lete diffusion of the chemicals
within the rood is about the same as that for the simple urea solution.
From this point on thn steps necess'r in Lhe entire bending and molding
process ar different from those suitable for Uralloy A. After the
treated wood is removed from the urea-formaldeh,- e solution, it is not
Iried, ut is prompt ly heated arnd bent in the web condition. If the
heating _nd ending are not don. promptly, res in will ),egin to form, and
the opl' ti-ity of the wood will decrease. ]hren this happe.-ns, reheatinL
.ill b. of no ail in resoftnin! the wood. IHeating is done by boill:.--
the tr ,Ad :,,ood in a 50 percent solution of urea in water at a tempera-
ture of 212 t 220' i1. for a period of 1R to 20 -minutes per inch of
thickss. e00o is then b.nt to the desired shape. Under such con-
ditions, the bending properties a,. ear to be as good as those of Uralloy
A. The wood is then drid while held to the bent form in an oven, kiln,
or simple e-posure to air, after which it is heated to about 3C' F.
in order to Pully polymerize the resin. Prolonged exposure to temperatures
of 150 F. or above also will set the resin sufficiently for most purposes.
,0hen the thermosetting process is used to resinify wooo for applicatiorls
ahich do not involve bnrding, such as for modification of hardness or mois-
ture resistance, the heating in urea and the bending are omitted.
Properties of the Product
As experimentation with the materials produced by the process has '"-, n
limited, all the properties of the new substances have been no means
established. In encal, however, te sts made thus far indicate that
the str;:.' th aualities ar uo t atrrial ly affected -, ttmp ratures up to
the boilin- point of wi t r. Uralloy C is st :ff.r tha'n nhrial wood, that
is, it does not be.d as much0 under the sm lio. It is consid ra4ly
harder. owe, v r, it c-n be readily worked with ordinary w -oodworking tools.
It tUks a high polish in urfed, UnIlk ur a trCatment, the urea-
alde 'de tre'Itm; nt nds to bleach r It tan to darken thc wood.
The formation of a wat<;r-resst:t urea-formaldeh'id re-sin in the fine
sw llin str ucture of ;r wood cll ,lls de stroys some of -h. natural
hygroscopic pro *rti es of the wood. The treatmeInt thus imarts true water
resistane<. :, rat mate amount of shr -. and sw lling in response to moisture ch-,:, ,,s are
cons ideralbly decreased compared vith those of normal wood.
i. t12(7 (r JJ5~~ I)
The mechanics of successfully bending wood treated by either mo-chod de .ands
largely upon good technique in handling properly designed ap aratus, as
mentioned earlier. As vet, research has not been carried far nouh
establish the shortest radius u-on which wood of a given thickness y be
bent Stock 1-1/2 inches thick has been bent through lc' deres o n
inside radius of 5 inches, and 1/4 inch stock was similarly bent on a
radius of 3/+ inch. h'hile this degree (3A+ inch) of upset is not neces-
sarily the maximum obtainable, it wil be noted that the ratio of the
length of the inside radius to the thickness of the stock is about 3:1
in both instances. It can be assumed, however, that thicker wood requires
a proportionately longer radius.
It has not yet been possible to establish whether the treatmt adds to
the resistance of wood to destructive insect and decay attack.
Research has been carried out on only a limited number of wood specis.
It appears, however, that the hardwoods respond better than do softwoods to
the plastici7'ng action of both processes. Of the hardwoods, oak and
sweeteum gi' better results than the other species tried. R t Lnif cation
of the wood Lb the urea-aldehyde solution probably cn n a nli d
with all hardw:oods arnd softwoods, although this has not been vrifid b
Urea, the synthetic ch mical which is the basis for this platicizing
treatment, is mbufactured co Trciall by combining liquid caron
dioxide and liquid *vmoroni- r pressure. It is comonly sol20 d-
crystailline form at ") to / a ton. It is not sold by the :ufact'rers
in quantities loss than 1sa poun'as, but smaller .atiti s can obtedir
from chemical and drg supply houses at a considarni ly hi her price,
It should be pointed out to those intre stid in underta.king co.:rci
applic ions of either proce -s tat exisin li mital tions upon doTt
sales of critical chemicals and rescue,. machinery, molds, dies, ec.,
caused by th. war rut e ta into consider .tion.
~o. lb al ~77 (revlzr d)
UNIVERSITY OF FLORIDA
3I 1262 08866 6390
3 1262 08866 6390