rrr[CT Or EXT[NIDINC HOT-PIIESS, UIR[A-I[SIN LUE[ WITIh
IY[ rLOCUIP ON STIENGTI1 AND ID)UPABILITY
01 THE 6LUE JOINTS
SMOL Of OW[Slky IRARY
UNIVERSITY OF FiCRioA
VIPC)IT IS ONE 0I A SERIIES ISSU[I)
AID TEll NATION'S WARI IPPI06IAM
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST PRODUCTS LABORATORY
In Cooperation with the University of Wisconsin
...J *** I
EFFECT OF EXTENDING HOT-PRESS, UREA-RESIN GLUE WITH RYE FLOR ON
STRENGTH AND DURABILITY OF THE GLUE JOINTS
By DON BROUSE, Engineer
Shortly after the introduction of synthetic resins of the urea type as
hot-press glues for woodworking, it was discovered that they could be mixed
with fillers or extenders that did not destroy the desirable working character-
istics or dry strength of the glue and that served to decrease the cost of the
zlue line considerably. One of the extenders commonly used at first was ryje
r'lour and its use persisted bo some extent although it was later re'-laced by
wheat flour in many cases.
Experiences in commercial practice and early tests demonstrated that
.':ints made with highly-extended, urea-resin glues by hot pressing possessed
h1-h dry strength and appeared suitable for use in many plywood operations.
The question was raised, however, as to the effect of the addition of an ex-
'-Mr.der, like flour, on the durability of joints that might be exposed to severe
r-vice conditions. Previous tests had demonstrated that joints made with pure
uree-resin glues by hot pressing were highly resistant to exposures involving
water at ordinary temperatures, to dampness, and to conditions favoring the
development of micro-organisms. The addition of fillers of an organic type
that mniht be affected by water and micro-organisms, however, might decrease
the resistance of the resin-glue joints to these agencies. This report,'
therei'ore, covers empirical tests to determine the effect of rye flour fil er
in a hot-press, urea-resin glue on the dry strength, initial water resistance,
,ld durability of the joints under severe conditions of exposure.
Six glue mixtures were used to glue plywood panels from which test
.- -cimens were taken In each case, the ratio of catalys- (or hardener) to
resin remained the same, but the amount of extender varied in the di( ferent
mixtures from none to twice the amount of resin. The extender used was rye
f ur and the glue was of the urea-ft'ormaldehyde type (Uformite 4:J "-._ the
Resinous Products & Chermical Company) marketed in a liquid form in which the
,-n ntration of the resin was reported to be approximately 70 percent T7'is
eV, .'ted concentration was not checked but flour extender was added on the
..suir.;ption that 100 pounds liquid resin contained approximately 70 ,cund.
dry resin. The mixture of resin as received, plus the amount of hard-n-r
recommPnded, was of a suitable consistency for spreading. When flour was
qdded, the mixture was thickened and addition of more water was required to
reduce the mixture to a suitable spreading consistency. In all the mixtures
containing flour, the attempt was made to add such an amount of water that the
consistency of the mixture was approximately the same as that of the straight
glue mixture containing no flour.
Following is a tabulation of the proportions used in preparing the
Parts by Percent flour
Constituents weight in the dry glue line
(based on amount of resin)
(1) Liquid resin 100 0
(2) Liquid resin 100 10
(3) Liquid resin 100 25
(4) Liquid resin 100 50
(5) Liquid resin 100 100
(6) Liquid resin 100 200
Of these glues, numbers 2 to 6 were prepared and used in February 1959,
while number 1 was prepared and used in January 1938. Glues number 2 to 6,
therefore, are all from the same shipment of resin while glue number I1 was from
a prior shipment of the same resin.
The preparation of veneer for gluing followed the same procedure whether
the gluing was done in 1938 or 1939. A quantity of 1/16 inch yellow birch
veneer, selected for smoothness, firmness, and straightness of grain, was con-
ditioned to approximate equilibrium with 30 percent relative humidity at 80' F,
(about 6 percent moisture content) before gluing. Eighteen 3-ply panels, 12
by 12 inches in size, were made with each glue and, after gluing, the panels
were stickered and again conditioned to approximate equilibrium with 30 per-
cent relative humidity before they were cut into test specimens.
The gluin- conditions were held nearly const'L.t throu.-h t th lui-
with the 6 different mixtures. The amount of ,lue spread varied from 1 to Uj
grar.s per square foot with most of the surfaces receiver, -,-- r s, re
foot (44 --ounds per 1,UJuI sure feet). The assembly period varied ver the
range from 10 to 30 minutes, but in most cases the .'-n..ls were crossed 15 minut
after they were spread. A gluing pressure of 2,-'- po.-inis ,fr s .'-re inch was
used throughout. The temperature of the platens w.is as roximately 2 F. and
the time in the press was 5 minutes.
From each panel, 30 plywood shear test specimens of conventional design
were obtained. Of these, 5 specimens were tested dry at a moisture content of
approximately 6 percent, and 5 were tested wet after soaking in water for 46
hours, fhe results of these tests were examined, all specimens from the ,r.nels
containing defective joints were discarded and specimens for further tests were
taken only from those panels in which the joints were well made. frne test
values of specimens of panels shown to be suitable for further testin.- were
tabulated according to the glue mixture used and averages were determined for
both wet and dry tests. These average values then served as controls and are
shown in the first two rows of Table 1. Even though the specimens glued with
the straight resin were prepared about a year prior to the remainder, the test
values compare reasonably well with those from the specimens glued with 10 Ler-
oent extension. The average wet test values of the two groups are well within
experimental error of duplicate tests on the same glue. In dry test values, those
specimens glued with the straight resin averaged 24 pounds per square inch higher
in strength and 20 percent lower in wood failure than those s,:ecimens ,'l'-ed vd th
10 percent extension. ven these variations are no greater than often occur in
successive tests on the same glue mixture. Since the control averages show no
discrepancies, specimens glued with the straight resin may be used as a : ,-is
for comparison even though they were glued prior to the others.
The remaining specimens of the panels selected for further tests were
arranged in sets according to the glue mixture used. Holes (3/S inch) vere
then bored near one end of each specimen. From each set of secimens glued
with one glue mixture, 4 groups of 80 specimens each vere selected at random
and each group strung on metal rods to facilitate handling throu.-h the :.. -
sure cycles (Fig. 1). Spacers were used between specimens to provide for
circulation of air and prompt change in moisture content in the .-lie test
One such group of specimens from each glue mixture was then soaked
continuously in water at room temperatures. A second group from e-ch r1ue
mixture was exposed continuously in a room where the temperature was main-
tained at S0" F. and the relative humidity at 97 percent. A third -roup from
each glue mixture was exposed to a repeating cycle that consisted of 2 dys
soaking in water at room temperatures followed by 12 day.s drv.rn- in an atmos-
phere maintained at SO"' F. and 30 percent relative humidit--, and a fourth
group was subjected to a repeating cycle that consisted of 2 weeks .: sure at
SO0 F. and 97 percent relative humidity followed by 2 weeks at -" F. and
percent relative humidity.
At the end of each interval of 2, 4, 6, 12, 10, 21, 73, and
months, 5 specimens were removed from each group and tested in the 1 d
shear testing machine. The test values of each such set of ; s-'-imens
were averaged and the results are recorded in the body of Table 1. Table 2
shows the same results calculated as a percentage of the dry test values for
the joints made with unextended glue. The testing schedule was so arranged
that, in the case of the "cyclic" tests, the testing was done at the end of
the "dry half" of the cycle. The specimens that were soaked continuously were
tested promptly upon removal from the water before they had time to dry out.
Specimens exposed in 97 percent relative humidity were likewise tested
promptly upon removal from the conditioning room before the moisture content
of the samples changed appreciably.
Results and Discussion
This report summarizes the results up to the end of the 36th month.
Sufficient specimens were provided to continue the test over a period of
some 7 years if the joints proved sufficiently durable. Consequently the
tests are being continued and further results will be available. At this
period of 36 months, however, certain significant trends are apparent that
can be changed but little regardless of the future course of the results.
Continuous exposure to 97 percent relative humidity
The effect of extending this hot-press, urea-resin glue with rye
flour was most pronounced in the tests involving continuous exposure to 97
percent relative humidity and 80 F., which is a condition favoring the de-
velopment of molds and wood-destroying fungi.
Glue joints made with the urea-resin glue extended with 200 percent
(based on the weight of the dry resin) of rye flour barely had strength to
hold the plies together at the end of the second month of exposure in 97
percent relative humidity (last column of Table 1). By the end of the l8th
month the last specimen in the group had failed completely. This compara-
tively low resistance to attack by dampness and micro-organisms is slightly
lower than shown by casein glue joints in previous tests under similar
Glue joints made with the urea resin extended with 100 percent of
rye flour had lost nearly all their strength by the end of the 12th month.
While several of the remaining specimens were still hanging together at the
end of the 18th month, the strength was so low that, of the 5 specimens
selected for test at that period, none was strong enough to give a reading
on the testing machine. As compared to casein glue joints under similar
conditions,1 joints made with this mixture of equal parts of urea resin
and flour were somewhat superior but not to a marked degree. Under the con-
ditions of the test, yellow birch veneer usually showed marked evidence of
rot in about two years and casein glue joints usually failed before the wood
had developed much evidence of decay.
-Exposure tests on plywood. Don Brouse. Mech. Engr. 60, 852-56, Nov.1938.
Joints made with the urea resin extended with 50 percent of rye flour
showed greater resistance to attack by micro-organisms thar. the joints muj-k-
with more extended mixtures, but at the end of 24 months the joint strength
was very low although most of the specimens were still holding together,
The drop in -.trength of joints with ?5 percent extension was or,,jual
throughout the test period and slower than with any of the hic-h-r dilutions.
By the end of the ;Oth month, however, the test values were low and the pointss
were no ln-er reliable although nearly all the specimens were holding to-
The specimens glued with a mixture containirg 10 parts of flour,
appeared :..nparatively clean and free from mold in contrast to the specimens
glued with more extended mixtures which were supporting considerable mold
growth on the surface (Fig. 1). At the end of 36 months, the glue diluted
with 10 parts of flour showed somewhat lower test values than were shocn by
the undiluted glue. Apparently the 10 percent dilution did not reduce to a
si-nificant degree the resistance of the glue to continuous high humidity.
Specimens glued with the unextended urea resin decreased gradually in
joint strength and at the end of 36 months the average test value was re-
duced to some 2S percent of the original dry test values (Table 2), with
zero wood failure. The resistance shown was considerably less than shown :y
hot-press phenolic glues in previous tests,1 which, at the end of 36 months,
averaged about 60 percent of the original dry joint strength with more than
9U percent of the failure in the wood. Joints made with hot-press, para-
formqldehyde-blood glues' averaged about 35 percent of the original dry
test values, with nearly all the failure in the wood.
Exposure to a repeating cFcle :,f 2 weeks in 97
percent relative humidity followed by 2 weeks
in 30 percent relative humidity
This exposure combines mechanical stress on the glue "'int (when the
-plies tend to swell or shrink with moisture changes) with exposure t- m- id
attack during the time- the specimens are under conditions favor'r. develop-
ment of micro-orgqnisms. If the glue line is completely resistant t, attack
by micro-organisms, this test is not so severe as a soaking-dry*r., cycle,
bu" if the glue is not so completely resistant to mold attack, it may :yve
more severe than a soaking-drying cycle in which test molds and 'urngi have
little chance to develop. In the case of joints made with hot-;-css, urea-
resin glues highly extended with rye flour, this exposure proved more severe
than the soaking-dryinr- cycles. In many cases the specimens failed -:. lately
and almost as rapidly as those exposed continuously to 97 percent relative
Specimens whose j.-ints were glued with a mixture containi:.:- -" per-
cent of rye flour had failed completely in this cycle at the end of the 6th
month. This rate of t'ai.lure is even more rapid than has been -'-served in
previous tests' with joints made with casein glue.
Joints made with a mixture extended with 100 percent of rye flour
failed completely in about 24 months. In previous tests, joints made with
water resistant casein glues have held together for some 12 to-15 months under
the same exposure cycle.
As compared with results on other glues in previous and similar tests,
the behavior of the joints with the 50 percent extension, with 25 percent exten-
sion and with 10 percent extension was somewhat better than that of joints
made with casein glues, but not so good as joints made with hot-pressed para-
formaldehyde-blood albumin glue.
The behavior of the joints made with the unextended resin glue resembled
that of joints made with hot-press paraformaldehyde-blood glue in previous
tests in the same exposure. They did not equal the average behavior of joints
made with hot-press, phenolic glues in the same exposure in previous tests.
Exposure to a repeating cycle of 2 days soaking
in water followed by 12 days drying in 30 per-
cent relative humidity and 80 F.
The effect of extension of the glue on the resistance of the joints too
this soaking-drying cycle was less pronounced than the effect on resistance
to exposure to conditions favoring mold growth. In spite of individual irreg-
ularities and inconsistencies, however, the resistance of the joints to wet-
ting and drying exposure apparently decreased slightly as the amount of ex-
tender was increased.
As compared to previous similar tests on other adhesives, the entire
group falls between the hot-pressed phenolic resins and the hot-pressed para-
formaldehyde-blood glues. Joints made with mixtures containing 200 percent
flour were slightly better than joints made with paraformaldehyde-blood glue
and joints made with the straight urea resin were not quite so resistant as
joints made with hot-pressed, phenolic-resin glue.
Continuous soaking in water at room temperatures
In general, continuous soaking in water proved to be the least
severe of the exposure tests used in this experiment, but the results
were reasonably consistent in indicating that the resistance of the joints
to continuous soaking decreased gradually as the amount of filler added to
the glue increased. The moisture resistance was high, however, even for
the highest dilution with starch.
At the end of 36 months of continuous soaking, joints made with the
unextended resin, with the mixtures containing 10 percent extension, 25 per-
cent extension, and 100 percent extension, were all sufficiently strong to
cause a high percentage of failure in the wood although the actual strength
test values were only some 50 to 62 percent of the original dry test values
uf the controls. The behavior of these joints compares well with that of
joints made with hot-press phenolic glues and with hot-press, paraformalde-
hyde-blood albumin glue in similar exposure in previous tests. Joints made
with mixtures containing 50 percent of flour and 200 percent of flour in
the dry glue line were still strong eno'-4h to cause -onsiderable failure in the
wood when tested at the ond of 3c, months of continuous s'akinr. Thv joints
made with 200 percent starch appeared considerably less resistant to continued
srctking than the joints made with the less extended mixtures, but they were
still more resistant than joints made with casein glues proved to be in earlier
Dry strength and initial water resistance
The effect of extension with starch on initial water resistance of the
joints was more regular than its effect on the resistance of the joints to
continuous soaking in water. This is shown in the first two lines of entries
in Tables 1 and 2. As the amount of extension was increased, the initial
water resistance of the joints fell off slowly and gradually up to an extension
of 100 percent. With 200 percent flour, however, the decrease in water
resistance was considerably greater.
The effect of extension on the dry strength of the joints was the least
pronounced of any of the effects investigated in this series of tests. '..T-le
the averages of the dry strengths showed a slow and gradual decrease as the
amount of extension increased, the differences between the averages as the ex-
tension increased were small and even the mixtures containing 200 -arts flour
showed high strength values and high wood failures. So far as dry strergth
was concerned, good joints resulted even when the glue mixture was highly
From the results of the tests up to and including thI 6th month of
exposure the following indications were noted:
1. The resistance of the glue joints to mold attack was most markedly
affected by extension with rye flour. Joints made with a mixture conta-n.nnr
200 parts of flour and 100 parts of resin proved scarcely equal to joints
made with casein glues in resistance to mold attack. The effect of 10 percent
flour was noticeable, although of minor importance, when the joints were sub--
is-ted to conditions favoring mold growth. Joints Made with unextended, urea-
resin glues did not prove so resistant to these conditions of exposure as
joints made with hot-press,-phonolic-resin rlues.
2. The effect of extension on the resistance of the joints to wttir.g
and drying cycles was less marked than the effect on resistance to conditions
favoring mold growth but, in general, the resistance fell off gradua-ly as the
amount of extension was increased. The reduction caused by 10 percent of flour
was slight and even the glue with 200 percent showed higher resistance than
would be expected of a casein glue.
3. Continuous soaking in water did r.not prove to be a severe e;.D:vre
for these urea-resin glue joints. Extensions of ]0 percent and 2= '.-rcent of
flour caused no decrease (as compared with unextended glue) that could be
detected by joint tests during 36 months. Joints made with mixtures con-
tainin- 200 percent of flour were definitely lower in their resistance to
continuous soaking, but still were more resistant than joints made with
casein glue in previous tests under the same exposure.
4. As the amount of extension increased, the initial water resist-
ance of the joints decreased somewhat more regularly than the resistance
to continuous soaking in water.
5. Within the range of extensions used in this experiment, the dry
strength of the glue joints decreased very slowly as the amount of extension
increased. While even a 10 percent extension, caused a slight decrease,
joints made with mixtures containing twice as much flour as resin were
still strong enough to develop a high percentage of wood failure on test.
6. Where maximum resistance to severe exposure is essential, par-
ticularly where the exposures involve conditions favorable to mold growth,
the extension of hot-pressed, urea-resin glues with flour should not exceed
10 percent and, preferably, should be avoided.
7. If the service conditions require only good dry strength and a
moderate degree of water resistance, the use of comparatively large amounts
of flour extender is permissible.
Table l.--CoBr.aratlve Etur 51iLtj c( joInt. cidr 1 i'.Z a rot- reused urea-reIin glue
usefd without extender and wIth different roi orton of an extender of
.3lue V I tiout : extended : Ett.nded : Lxtended : xtended : Extended
I extender : with witt with w ith I with
Time .. 1 10 : 25 50 1 100 : 200
of s percent i percent I percent percent percent
Exposure : rye flour: rye flour rye flour rye flour : rye flour
-- -- Average test valueal
None controll values)
t0 i 07-61 1483-83 '458-89 1435:88 1 440.90 1 429-83
S ,89-9 4.78-9 1 55-90 14 26-71 405- 0 i 261- 8
Boaxeod oontinuouely In water at room tepsqrature:
2 444- 81 1 426- 99: 317- 66 376-53 : 366- 79 9 253-12
l 441- 100 o 42- 1 1 359- Z79 2 89-1 354- 1 235-13
6 401- 96 565- 59 s350-1 S 297- 5 ,o.00- 1, 18- 2
12 1003 379- 84 : 50- 8O 322-61 1 22 :27-1;
18 I 0 t 328-100 1-100 2&45-20 6- 52 20r4-1
24 516-100 295-100 11- 92 24r-68 240- 80 t 17-22
1 298-100 27 -1001 267-100 : 275-88 218- 82 1 172-
5 315- 091 26 96 1284- 90 1245-48 21-100 1 176-
Exposed continuously to 97 percent relative humidity and 800F.1
U 4347- 3 408- 43 ( 284-16 231- 21 0-0
S3- 5 ; 337- 2 : 237-20 202- 1 25-0
6- 4 258- 20 235- i ; 21-19 149- lz6 13-0
1 509- 41 1290- 40 25- 0 91 0 1 79- 0k I 0-0k
18 5 309- 0 222- 36 140- 0 t 90- 1 0- 1- 1 o(18)
2k 155- 0 225- 20 1 0 18- .............
30 s 19- 170- 20 79- 0 1 0 1 ............
36 144 0 108- 0 : 49-0 32- 0 28- 0 .............
X2oeed to a reneating cycle that cona.eted of day.e ,o.ing in wer r at room
teperature followed by 12 day dryin at 80 and 30 percent relative
247- 36-3- 76 ; 33-39 1 351- 19 330-1?
: 4442- 5 397- 42 410-23 : 378- 23 3 57-1
6 t 446- 1 1 40-68 390- 22 t 447-'5 : ti6-142 36f'4-342
it : 436- 12' 357- 41 345- 26 : 083- 315-1 24 26 4-20
i1 511- 2 5 354- 28 S 331- 3 : 289-18 ; 292- 22 310- 1
24 : 40- 81 3345-100 214- 40 : 217-20 157- o t 166- 0
30 5 362- 0 : 331- 40 : 282- 40 ; 31Z-19 24o- 20 209-41
36 394.- 0 : 299- 5 : 178- 19 281-20 :272- 20 16.9-20
jxnomed to & re ~ating cycle that conalsjed of 2a weeks %n 8 F. Ind 97 Dercent
relative h,,-l lt followed bY 2 weaen in 800 F. and 3Q percent relative
2 : 514- 80 45- 60: 392- 31 3 419-30 ; 35- 26 134- 0
14 524- 251 3b8- 49 371- 00 1 321-23 : 220- 30 : o- Ot
6 : 476- 24 : 35- 11 : 329- 2 : 300- 0 : 181- 0 t O():
12 : 376- 21 : 50- 0 : 20C- 2 : 124- 0 : 181- 20 : .............
18 410- 12 ;0o- 20: 184- 0 1- 0 0 : l46- 8 .............
24 t 248- 0 : 309- 21 : 183- 0 : 111- 0 0(M4): ............
30 : 235- 0: 38- 0 124- 0 ........... : .............
36 : 187- 0 60- 0 : 91- 0 : 6- 0 ........... : .............
1n each pair of values, the first represents average
Joint strength in pounds per square inch, the second
represents wood failure in percent.
Eoach value is an average of 100 specimens, 5 from each
of 20 panels.
Specimens tested when in approximate equilibrium with
30 percent relative humidity.
-Bpeoimens tested wet after soaking for 48 hours in water
at room temperatures.
59aoh value Is an average of five speoimens.
figuress in parentheses indicate the month during which
the last speoimen In the group failed completely.
'In the five specilens taken for test the Joints were
still holding the plies together but the strength was
so low that the speoimens could not be gripped In the
testing machine without failure.
Table 2.--Data from table 1 showing Joint strength as a percentage of the average dry
test value of Joints made without the use of extenders
Glue : Without : Extended s Extended Extended Extended : Extended
extender : with : with with : with with
Time 1 10 25 1 50 100 200
of tI 3 percent S percent : percent : percent 3 percent
Exposure ;3 1 rye flour: rye flour : rye flour : rye flour ; rye flour
------ ----------------------I_t -----------------------------------------
-oth Average teat values1
None (control values)
Dry 100- 61 95-83 90- 89 3 87 87-90 t 85-83
Weti 2 0 96- 96 9t 94 '3 P 89- 90 : 86-71 : 80-60 1 51- 8
Soaked continuously in water at room temDeraturEMA
S88- 81 1 84- 99 36 66 t74-51 72- J? 0-12
8-100 1 $1- 81 1 0 69-5 to I
66 72- 59 69-196 t4 -9- 62 6-.
12 8 t-10oo ( It : 69-1 1 1 69-22 s L
18 67-100 5-100 R6-100 :4820 48- 52 : 0Q19
24 3 62-100 t 58-100 61- 92 t49-68 t 47- 80 127-22
30 s 59-100 : 55r-100 53-100 5488 :147- 82 34-29
36 62- 96 : 52- 96 t56.- 48 4 55-100 35-64
Exposed continuously to 97 percent relative humidity and 800 r
2 94- 99: 93- 83 : 80- 43 : 56-16 46- 21 o-o
4 77- 45 : 80- 59 : 66- 23 : 47-20 : 4o- 19 5-0
6 2- 4 51- 20 : 46- 1 : 42-19 30- 16 : 3-0
12 61- 41 57- 4o : 42- 0 : 19-0 16- O 0-0 o-o
18 : 61- 0 : 44- 36 : 28- 0 : 18- 1 : 0- 0- : 0(18)-
24 : 31- 0 : 44- 20 : 27- 0 g 5- 0 : 4- 0. ...........
30 39- 0 : 34- 20 :16- 0 : 3- 0 : 0- : ...........
36 l 28- 0 : 21- 0 : 10- 0 : 6- 0 : 6- 0 ...........
Exposed to a repeating cycle that consisted of 2 days soaking in water at room
temperatures followed by 12 days drying at 80 F. and 30 percent relative humidity
2 s 95- 92: 74- 45 75- 76 ;68-39 69-19 :65-19
a-& : 77-59 : 78- 42 81-23 : 75-23 66-14
88- 1 : 91- 3 Z7- 22 88-57 72- 42 74-4:
12 86- 12 : 70- 41 : 68- 26 61- 4 224 -c
:101- 82 S 70- 28 65- 3 3 7-18 58- 22 : o- 1
87- 81 -1'* :4-40 31?0 31-0
71- 0 t 6-4o n -4 : 2-19 : o 14-41
: 78- 0 : .:-" 5 : .'- 19 : 55- 2o : 4- 20 33-2o
-.,Zed p repeating pycle truLt. :.s''ed :f 2 weeKs in EO0 F. and 97 percent
"di Y n-',, "10 n _
-" ..'-,idlty fo lcred b. *" ,- n .., F. 1nd ;':' e, -t relative humidl,'.-
2 ; 101- 80 : 86- 60 77- 1 :83-30 :64- 26 : 26- 0
4 t l 2 73- 7- 0 : 63-23 : i- 1C O- 0
6 : 94- 4 : 70- 1 o,3- 2 ; 9- 0 : Ib- '0 t c,'(6'
12 t 74- 21 t 49- 0 40- 2 : 24-0 o 36- 20 t...........
18 I 81- 12 : 57- Z0 A6- 0 21-0 C !, ...........
2 t 49- 0 .- b : )t- 0 22- 0 24, : ...........
S 46- o : 65- 30 24- o 04- 0 ........... .......... .
S37- o !?- 0 18- 0 12- 0 ........... : ...........
each pair of values, the first ,epre-sents average Joint strength e,- ed
B. rr *". of the or1.:ral dry teat i.... i of !.p. made without
exh -t. tre 0,eco -< = actual wood failure in percent. The wood
....re values are he same as in table 1.
--_ ue is an average of .r-., sp.we,-,, 5 from each of 20 ,;e-.-i8
-:..: ,s tested in a )-rl.mate equilibrium with 50 percent relative humldlty,
.e -.. eteed wet after oaing 4- hours in water at room tures
value Is an -. ..'- of five ,c'cmr -,,
ein parentheseB indicate the month 4' -. which the last Ib.-.ir
In e group failed _. w, Pet1-n. .
th five speclens taken for te the Joints were tlli _.: t bdn the plies
r t the. was eo low that the specimens could not be grti'.,ed
n e teet ...mah without failure.
2. 4 .F
I '-A I.
r I N
WIII 1111-3 4E7
us '** .^ ^*^^- .' -..
Figure l.--Arrangement of specimens for handlir.- thrc .-h
exposed specimens shown are, of course, from
merely to show the normal aF:nr-ance : te t
comparing the :.:r.!ition of the ferventt -.-
tion should be paid primarily to the c tic
z u 40771 F
e,: ure cycles Thu tn-
ither test and are included
means s before ,: ure. In
cf *:,x: sed : atter-
>f the Indi~iual s"-- i .ns
UNIVERSITY OF FLORIDA
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