i2J21-C'
DURAHLITY OF GLUE JOINTS 131ETWEIN
MAXUS OF COMMEG AND Of
F OREST PRODUCTS LIBRARY
COMPREG AND WOOD FOREST RESEARCH LABORATORY
Information Reviewed and Reaffirmed OREGON STATE UNIVERSITY
June 1960
No. 1536
Illflllllflllllll
FOREST PRODUCTS LABORATORY
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
MADISON 5, WISCONSIN
hn
ooperatIon with the University of Wisconsin
DURABILITY OF GLUE JOINTS BETWEEN BLOCKS OF COMPREG
1
AND OF COMPREG AND WOOD –
By
HERBERT W. EICKNER, Engineer
Forest Products Laboratory,? Forest Service
U.S. Department of Agriculture
Gluing studies on compreg at the Forest Products Laboratory have shown
that, with the proper glues, gluing conditions, and surface preparation,
joints of high initial strength can be obtained between compreg blocks
and between compreg and wood.
With this finding established, a study was begun to determine the durability of such joints when aged under controlled conditions of moisture
and temperature. Four types of resin glues were used to prepare the
joints between compreg and compreg and between compreg and wood blocks,
and these joints were subjected to six different exposure conditions
and tested periodically t6 detect changes in joint strength. This
first report presents the results of shear tests of the glue joints after
periods of exposure up to 1 year. Similar tests are planned at the end
of 2, 4, and 5 years of exposure to the same conditions.
Materials
Glues
The following glues were used in preparing the joints:
Glue A, a room-temperature-setting urea resin.
Glue B, an alkaline, intermediate-temperature-setting, phenol resin.
Glue C, an acid, intermediate-temperature-setting, phenol resin.
Glue D, a room-temperature-setting resorcinol resin,-3
1This 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. Original report dated August 1945.
?Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
•While the resorcinol glue used in these tests is recommended for use at
room temperatures, the joints for these.tests were cured at 140° F.
Report No. 1536
-1-
Compreg
Two types of compreg were employed in this study; one, a commercial compreg
impregnated with an alcohol-soluble phenol resin, designated "compreg Al"
in this report; and the other, impregnated with water-soluble phenol resin,
made in part by a commerical producer and designated "compreg W" in
this report, and in part by the Forest Products Laboratory and designated
herein as "compreg WL."
All of the compreg used conformed to Army Air Forces Specification
No. 15065 dated June 10, 1942. The specific gravity of compregs W and
WL Vas 1.38 and that of compreg Al was 1.32. The resin content was
between 25 and 30 percent.
The procedure for the preparation of compreg WL at the Forest Products
Laboratory was as follows: One-sixteenth-inch yellow birch veneer at a
moisture content of 5 to 7 percent was immersed in a water-soluble, phenol
formaldehyde resin, and air pressure of 50 pounds per square inch was
applied for 16 hours to obtain thorough impregnation. Following a diffusion and air-drying period of about 8 hours, the residual solvent was
removed by drying for 16 hours at 130° F. The veneers were then assembled
in groups of 16 piles with the grain direction of all plies parallel. The
assembly was placed in a hot press under a pressure of 1,000 to 1,200
pounds per square inch and cured at 320° to 330° F. The time under
pressure for a panel of 5/8-inch final thickness was approximately 45
minutes.
Compreg W was apparently similar in most respects to the Forest Products
Laboratory compreg, though made with a different water-soluble resin
without pressure treatment during impregnation but with a longer soaking
and diffusion period to achieve a high resin content with uniform distribution. The procedure for removing solvents included 1 day of air
drying at room temperature and 1-1/2 hours at 150° F.
Compreg Al was made of 10 plies of 1/10-inch sugar maple impregnated
with an alcohol-soluble phenol resin, but other details of its preparation are not known.
The compreg panels used in this study had a thickness of approximately
5/8 inch. They were sawn into 5- by 12-inch blocks, with the grain of the
veneers parallel to the long dimension, and were conditioned for more than
2 weeks at a relative humidity of 30 percent and at 80° F. before being
surfaced on both faces with a metal milling machine to a thickness of
about 1/2 inch. Although the resulting surfaces were not perfectly flat
(light could usually be seen through the space between a pair of the
blocks laid together), the surfaces could be forced into contact by light
pressure.
Report No. 1536
-2-
Wood
Sitka spruce and sugar maple, selected for density and freedom from
curly grain, sloping grain, mineral streaks, knots, or other defects,
were used in this study. The average specific gravity was 0.38 for
the Sitka spruce and 0.68 for the sugar maple, based on ovendry
weight and volume.
The wood was cut into blocks 7/8 by 5 by 12 inches in size, with the
grain lengthwise of the block. These blocks were conditioned for at
leant 1 week at 30 percent relative humidity and 80° F. before being
planed to approximately 3/4 inch. The blocks were glued to the compreg
soon after being surfaced and before there was any noticeable warping.
Test Procedure
Gluing Conditions
Glue spread.--A light double spread was used with a total of approximately
15 to 20 grams of wet glue per square foot of glue joint. In all cases
there was ample squeezeout.
Assembly time.--A closed assembly time of 10 minutes was used with glues
A, C, and D. With glue B longer assembly times found desirable by
preliminary experimentation were used--a 45-minute open followed by a
30-minute closed period for compreg-to-wood joints, and a 90-minute
open followed by a 30-minute closed period in gluing compreg to compreg.
Curing time and temperature.--The joints glued with the room-temperaturesetting urea resin were pressed for 18 to 20 hours in a room at a
temperature
75° F. Those made with the acid, intermediate-temperaturesetting phenol were cured in a kiln for 24 hours at 160° F., and
those made with the resorcinol glue in a kiln for 20 hours at 140° F.
Since the alkaline, intermediate-temperature-setting phenol, glue B,
required ;a higher curing temperature than could conveniently be
obtained in the dry kilns, the joints made with this glue were cured in
a high-frequency electric field. The temperature within the joints
was brought to 250° F. and held for a period of about 20 minutes.
Pressure.--Pressures of 150 pounds per square inch were employed for
gluing compreg to Sitka spruce, and 250 pounds per square inch for
gluing compreg to sugar maple and to compreg. In the high-frequency
gluing of compreg to sugar maple with glue B, however, a pressure of
200 pounds per square inch was used.
Conditioning.--All joints were conditioned for at least 1 week at 30 percent relative humidity and 80° F. before being cut into specimens and
placed in the exposure racks.
Report No. 1536
-3-
Material Combinations
The various compregs, glues, and species of wood were tested in 20
different combinations as follows:
Compreg
WL
WL
W
Al
Al
Al
WL
WL
W
Al
Al
Al
WL
WL
W
Al
W
W
Al
Al
Glued to--
Glue Used
Spruce
Maple
Compreg W
Spruce
Maple
Compreg Al
Spruce
Maple
Compreg W
Spruce
Compreg Al
Maple
Spruce
Maple
Compreg W
Compreg Al
Compreg W
Maple
Compreg Al
Maple
A, urea resin
"
"
A,
If
/I
A,
A,
/I
//
A,
II
If
A,
If
II
B, alkaline phenol
//
If
B,
If
II
B,
If
/I
B,
/V
Of
B,
If
/I
B,
C, acid phenol
"
C,
"
"
C,
"
C,
"
"
D, resorcinol
I/
D,
If
D,
D,
•"
Number and Distribution of Test Specimens
Twenty-four 5- by 12-inch joints were prepared for each of the 20
combinations. Ten standard block-shear specimens, each with a single
shear area of 3 square inches, were cut from each joint. Each combination
was thus represented by 240 block-shear specimens. Thirty specimens
from each combination were selected for the unexposed controls. The
210 specimens remaining for each combination were distributed equally
among 6 exposure conditions in such a manner that only 1 or 2 from
each joint would be subjected to each exposure condition. The scheme
of distribution also provided that only one or two specimens from each
joint would be tested at each test period after having been subjected
to one of the exposure conditions. Five specimens of each combination
were tested after exposure to each condition for periods of 4, 12, 24,
and 52 weeks, and it is planned to test an equal number after 2, 4,
and 5 years.
Report No. 1536
-4-
Exposure Conditions
The exposure conditions to which the specimens were subjected, and the
condition of the specimens at the time of test, were as follows:
J (1) Continuous exposure to 80° F. and 30 percent relative humidity.
(Specimens tested dry.)
V (2) Continuous soaking in water at room temperature. (Specimens tested
wet.)
j (3) Continuous exposure to 80° F. and 97 percent relative humidity.
(Specimens tested moist immediately upon removal from the high
relative humidity.)
(4) A repeating cycle consisting of 2 weeks at 80° Rand 97 percent
relative humidity, followed by 2 weeks at 80° F. and 30 percent
relative humidity. (Specimens tested dry, immediately after removal
from 30 percent relative humidity.)
J(5) Continuous exposure to 200° F. and 20 percent relative humidity.
(Specimens cooled to room temperature and tested dry.)
(6) A , repeating cycle consisting of 1 day at 200° F. and 20 percent
relative humidity, followed by 1 day ,* -25° F. (Specimens brought
to room temperature and tested drY.).tt '
Summary of Test Results
The results of the joint tests made after exposure up to 1 year are
summarized in tables 1, 2, and 3. In view of the incompleteness of the
data, the , following general summary is offered with the caution that the
conclusions may be changed with more complete results from longer exposure.
Strong original joints were obtained with all four glues and with all
combinations of compreg, maple, and spruce.
The average compreg or maple failures were 70 percent or over in the
control joints of compreg to compreg and compreg to maple bonded with the
urea and phenol glues. The joints glued with the resorcinal glue broke
with lower compreg and maple failures, 39 to 51 percent, but the strengths
of the joints were not much lower than with the other glues.5
4
—On Sundays, the specimens were not changed but allowed to remain for
2 days.
5
—later observations indicate that higher compreg and wood failures might
have been attained with this resorcinal glue with a longer closed
assembly period than 10 minutes.
Report No. 1536
-5-
The control joints of compreg and spruce broke when tested in shear,
with the failure almost wholly in the spruce.
In general, the strength of the compreg-to-compreg joints was higher
with compreg Al, impregnated with alcohol-soluble resin, than with
compreg W, impregnated with water-soluble resin. This difference can
be partially explained by the use of maple veneer in compreg Al and birch
veneer in compreg 14,2 but the difference between the strength of compreg
W and Al compreg-to-compreg control test joints is too much to be completely explained in this way.
As to the order, in general, of durability of the compreg-to-compreg and
compreg-to-wood joints, it would appear from the data of tables 1, 2, and
3 that the alkaline, intermediate-temperature-setting, phenol-resin glue
joints, and the room-temperature-setting, resorcinol-resin glue joints,
cured at 140° B., rank together in the position of highest durability; the
acid, intermediate-temperature-setting, phenol glue joints are second;
and the room-temperature-setting, urea-resin glue joints last.
There was some reduction in strength in certain of the compreg-to-compreg
joint combinations even under the mild exposure conditions of 80° F. and
30 percent relative humidity, particularly with the urea and the acid
.phenol glues.
Continuous soaking resulted in lower strength values for all joints only
in part due to wetting of the wood and compreg, for the loss in strength
in (certain joints was greater than could be accounted for in this manner.
The joints glued with the room-temperature-setting, urea-resin glue
particularly showed marked decrease in quality after soaking, although the
amount of decrease varied with different combinations. Joints of compreg
to compreg bonded with room-temperature-setting urea resin had very little
strength after 1 year of soaking, but the broken joints revealed a thin
layer of compreg over much of the area that failed in shear. The joints
consisting of two blocks of compreg Al bonded with the acid intermediatetemperature-setting phenol lost strength conspicuously but also showed
a high percentage of the thin type of compreg failure.
The continuous high-humidity exposure was similar to the continuous soaking in that it lowered the strength of all joints by wetting the compreg
and wood. In this exposure also, the joints bonded with the roomtemperature-setting urea resin and the joints of compreg Al blocks bonded,
with the acid intermediate-temperature-setting phenol were particularly
reduced in quality.
6
other studies at the Forest Products Laboratory, it has been shown
that most of the strength properties of compreg depend on the specific
gravity to which it is compressed rather than on the species. Shear
strength at right angles to the direction of compression is, however,
an exception to this generalization. Woods that are high in strength
in shear parallel to the grain produce compregs that are proportionally high in this strength property.
"-In
Report No. 1536
-6-
The effects of alternate exposure for 1 year to high and low humidity
varied with the combinations. With the wood-to-compreg joints, in general,
the wood failures did not fall off and the strength decreased but little;
whereas with the compreeto-compreg joints there was considerable decrease
in strength, save for one combination bonded with resorcinol resin,. often
without appreciable lose in compreg failure.
Continuous exposure to 200° F. proved to give the most severe test of
joint durability. Under this condition, the urea-resin glue joints lost
most or all of their strength and percentage of failure in the compreg or
wood. The compreg-to-compreg joints glued with the alkaline intermediatetemperature-setting phenol and the room-temperature-setting resorcinol
retained strengths of more than 2,000 pounds per square inch after 1 year
of this exposure, but those glued with the acid phenol were greatly
weakened. In spruce-compreg combinations bonded with both the alkaline
and acid phenol glues, joint strengths were low and wood or compreg
failure high after exposure to 200° F. for 1 year.
The alternate low- and high-temperature cycle produced results similar to
those from exposure to continuous high temperature.
The frequent occurrence in the data of tables 1, 2, and 3 of strength
values of joints after exposure that are definitely lower than the corresponding control strength values but with about the same amount of wood or
compreg failure, gives rise to the surmise that considerable unrelieved
stress develops within the materials during aging under the exposure
conditions employed in these tests.
Report No. 1536 -7-
. 2-11
1
Table 1.--Durability test results on Sitka spruce-compreg joints glued with three resin glues -
Glue C
Acid intermediate: temperature phenol
:
Period of : :
Compreg WL
Compreg WL
exposure s
Compreg Al?
t
Compreg WLE
Compreg Al
:
r
.
•
: Shear
s Wood
: Shear
1 Wood
: Shear
s Shear
: Wood
1 Wood
s Wood
1 Shear
: strength t failure : strength t failure s strength t failure e strength : failure : strength : failure
s
:
t
:
s
g
s
. t
s
s P.s.i. t Percent : P.s.i. t Percent : P.s.i. g Percent : P.s.i. g Percent : P.a.i. s Percent
t
t
Control
Glue A
Room-temperature urea resin
t
t
s
1,530 s
99
:
1
:
1,465 t
t
:
95
Glue B
Alkaline intermediate-temperature phenol
:
:
g
1,413 8
96
t
$
t
:
t
e
1,402 :
96
s
1,306 t
:
:
e
:
1,409 :
1,458 1
1,896 :
1,568 :
94
99
100
100
1
t
:
:
95
1,210 s
89
1,001 t
1,083 :
94
1,212 : 100
:
:
870 :
469 :
846 :
921 :
98
100
92
97
:
:
99
702 :
99
791 :
627 s 100
637 : 100
1,082 :
1,045 s
1,208 :
1,007 g
98
100
100
100
:
s
:
s
1,026 t 100
99
866 :
98
1,122 :
1,007 s 100
96
80° F. and 30 percent relative humidity
4 weeks
12 weeks
24 weeks
1 year
s
t
t
s
1,224 t
1,031 s
1,623 s
964 s
93
97
96
95
s
s
$
1
1,250 :
1,529 :
1,318 t
1,210 :
97
100
96
99
s
t
:
t
1,249 :
1,195 1
1,585 :
819 :
100
99
93
96
Continuous soak
4
12
24
1
weeks
weeks
weeks
year
829 s
443 t
904 :
341 t
834 t
685 :
791 e
699 :
63
22
60
31
84
66
82
77
:
:
1,007 :
768 :
755 :
551 :
99
94
100
80
80° F. and 97 percent relative humidity
4 weeks
12 weeks
24 weeks
1 year
t
:
g
8
1,316 :
917 t
1,116 t
486 e
97
92
66
70
s
t
t
s
1,150 :
1,027 t
1,187 s
877 1
100
97
96
79
:
:
:
s
1,099 :
1,020 e
1,096 s
955 :
97
100
100
93
:
s
:
g
80° F. and 97 percent relative humidity for 2 weeks and 80° F. and 30 percent relative humidity for 2 weeks
4 weeks
12 weeks
24 weeks
1 year
t
t
:
:
1,231 s
1,011 t
1,308 t
868 t
99
96
98
89
$
t
s
t
1,199 1
1,741 t
1,371 s
1,160 8
98
98
100
96
:
:
:
1
1,415 :
1,123 :
1,677 s
941 8
100
100
97
95
:
:
1
$
1,563 :
1,273 t
1,404 g
1,395 :
99
1,083 :
86
938 s
900 :
91
918 : 100
97
100
98
100
:
:
96
99
98
100
:
:
:
:
398
538
403
381
95
100
100
100
:
:
:
s
:
589 : 100
94
392 :
152 : 100
384 t 100
t
t
:
Continuous 200° F. and 20 percent relative humidity
4
12
24
1
weeks
weeks
weeks
year
1
:
s
729 t
623 t
574 :
103 8
98
100
45
13
:
t
t
542 g
538 :
269 t
205 :
90
64
18
17
3
:
657 t
555 :
998 :
277 :
100
100
90
99
s
:
:
:
1,130 :
800 I
842 :
467 :
97
t
: 100
8 100
92
:
Alternate 1 day at 200° F., and 1 day at -25° F.
4 weeks
12 weeks
24 weeks
1 year
t
s
g
t
s
479 s
535 s
566 t
122 3
g
90
100
81
1
42
:
703 :
434 :
363 :
259 :
:
93
89
70
41
:
:
:
:
:
787 :
630 :
939 :
620 :
s
99
100
97
94
8
:
:
:
g
1,552 :
962 :
942 :
417 :
e
-Each result for the control tests is the average of 30 specimens, and each result for the exposure tests
is the average of 5 specimens.
?Compreg made of sugar maple impregnated with alcohol-soluble phenol-formaldehyde resin.
!Compreg made of yellow birch impregnated with water-soluble phenol-formaldehyde resin.
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