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FOREST PRODUCTS LIBRARY
FOREST RESEARCH LABORATORY
OREGON STATE UNIVERSITY
VARIATION IN MAXIMUM ALLOWABLE
ASSEMBLY TIME WITH AGE IN THE
POT AT TIME Of SPREADING
FOR FOUR RESIN GLUES
Information Reviewed and Reaffirmed
March 1956
No. 1546
ri
111,111.,„!„,.
FOREST PRODUCTS LABORATORY
UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
MADISON 5, WISCONSIN
in Cooperation with the Univers i ty of Wisconsin
VARIATION IN MAXIMUM ALLOWABLE ASSEMBLY TIME
WITH AGE IN THE POT AT TIME OF SPREADING
FOR FOUR RESIN GLUES 1—
By
W. Z. OLSON, Technologist
and
H. D. BRUCE, Chemist
Forest Products Laboratory, ? Forest Service
U. S. Department of Agriculture
Introduction
As a thermosetting resin glue stands in the pot after mixing, the maximum assembly timel at which strong joints can be made is considered
to become shorter. It is generally assumed that the pressure should be
applied well before the end of the working life when the glue in the pot
becomes no longer spreadable. It is not known definitely, however, how
the permissible assembly time should vary as the glue ages in the pot,
or how the time of application of pressure should conform to the end of
the working life of the glue. This study was undertaken, therefore, to
obtain experimental information on this subject.
-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. Original report published in 1946.
?Maintained at Madison, Wis. , in cooperation with the University of
Wisconsin.
-Assembly time in gluing is defined as the period between spreading the
glue and applying the pressure. The assembly is called "open" if
the surfaces to be joined are exposed to the air during this period or
"closed" if the surfaces are together.
Report No. 1546 -1-
Agriculture -Madison
Two room-temperature-setting resorcinol-resin glues (designated A and
B in this report), an intermediate-temperature-setting phenol-resin
glue (designated 13 ), and an acid-catalyzed intermediate-temperaturesetting melamine-resin glue (designated M) were selected for study as
representative of types of current importance in the fabrication of
aircraft.
Joints were made with hard maple blocks, spreading each glue at various
intervals between the time of mixing and the end of the working life, and
allowing various periods of assembly, both open and closed, before applying the pressure and curing the glue. The quality of the joints so
made was ascertained by testing specimens in shear.
Procedure
Hard maple blocks, 7/8 by 2-1/2 by 12 inches, were selected for
straightness of grain, freedom from defects, and specific gravity ranging between 0.63 and 0.70 based on weight and volume when oven-dry.
The blocks were conditioned to a moisture content of about 12 percent,
surfaced on both sides to a thickness of 3/4 inch just before gluing, and
glued together in sets of three laminations each.
Three assembly temperatures were chosen for this study, 70°, 90°, and
105° F. , and the working lives of the glues, mixed from ingredients at
70° F. , were determined at each of these temperatures; the end of the
working life was considered as that time at which the glue could no longer be smoothly spread with a stiff bristle brush. When the glue was
spread and the joints assembled, the gluing room was at 70°, 90°, or
105° F. The ingredients were always at 70° F. before mixing, and the
mixed glue was allowed to adjust itself to the temperature of the room
when the room was at 90° and 105° F. For each experiment, two duplicate batches of glue of 100 grams each were mixed according to the
manufacturer' s directions. One batch was observed frequently for a redetermination of the length of the working life; the other batch was used
for gluing the joints. When the glue was spread, the wood was always at
the gluing room temperature. The glue was applied by brushing at three
intervals during the working life of the glue, namely 5 minutes after
mixing the glue, when 50 percent of the working life as previously determined had elapsed, and when 75 percent of the working life had elapsed.
The glue was applied to only one of the two joining surfaces. The
spreading rate was about 65 pounds of wet glue per thousand square feet.
Report No. 1546
-2-
Various arbitrarily chosen periods of open and closed assembly were allowed after spreading the glue. Longer and longer assembly periods
were chosen until shear tests of the cured joints showed a decided falling
off in strength.± At the end of each assembly period a pressure of 100
pounds per square inch was applied to the joints.
The joints made with resorcinol-resin glues were allowed to cure, under
pressure, at the same temperatures as those used in assembling the
joints, for 24 hours at 70° F. , 16 hours at 90° F. , or 6 hours at 105° F.
The joints made with phenol-resin glue were cured under pressure for
24 hours in a chamber having a temperature of 180° F. The joints glued
with melamine-resin glue were cured under pressure for 24 hours in a
chamber having a temperature of 140° F. In each case the humidity in
the curing chamber was controlled to keep the moisture content of the
wood at about 12 percent. After removal of pressure, the joints were allowed to condition at 80° F. and 65 percent relative humidity at least 6
days before they were cut into specimens.
Ten specimens were cut from each glued assembly, and at least 10 specimens were tested for each assembly condition. Usually 20 or 30 specimens, fr6m 2 or 3 glued assemblies, were tested for each assembly
period.
Results
Both shear strengths and percentages of wood failure were recorded for
each specimen tested, but attention has been given in the following
presentation principally to the shear strength data. The' average wood
failures were reasonably high for all conditions, except those that had
such long assembly time that the shear strengths fell well below 2,800
pounds per square inch. To illustrate, for those specimens that tested
between 2,800 and 3,000 pounds per square inch the wood failure percentages averaged as follows: 48 for glue A, 34 for glue B, 65 for glue
M in both open and closed assembly, and 47 for glue P in open assembly.
For glue P in closed assembly, the average percentage wood failure of
all specimens that had 5 days or less assembly time was 49 percent.
4
—In the case of the phenol-resin glue, P, it was found that periods as
long as 7 days in closed assembly at 70° and 90° F. had not caused
a decided drop in shear strength, but longer assembly periods were
not investigated.
Report No. 1546
-3-
The thickness of glue line was measured with a microscope on representative joints and inspected on all joints. In general, the lines were
thicker the higher the assembly temperature and the longer the assembly
time; but, even of those joints with such a long assembly time that the
strengths were low, the glue lines did not exceed 0.010 inch in thickness.
Closed Assembly
Resorcinol-resin glues. --In figures 1 and 2 the average shear test values
of the maple block joints prepared with the resorcinol-resin glues A and
B in closed assembly have been plotted against the age of the mixed glue
at the time the pressure was applied to the joints. Each point in the
figures represents a set of specimens made with a particular assembly
time. For example, in preparing the joints represented by point a in
figure 1, the glue was brushed on the blocks 5 minutes after mixing;
then 315 minutes passed in closed assembly, and the pressure was applied 320 minutes after the glue was mixed.
An interesting observation from figures 1 and 2 is that joints were obtained of as high strength as can ordinarily be made from maple of the
density employed, even though the pressure was applied some time after
the glue in the pot could no longer be spread.
The principal conclusion from these figures is that, for joints of at least
2,800 pounds per square inch by these resorcinol glues within the conditions of the experiment, the interval between mixing the glue and pressing the joint was approximately a constant. This constant varied with
the glue and the assembly temperature. The value of the constant for
glue A was about 375 minutes for 70° F. , assembly temperature, about
150 minutes for 90° F. , and about 90 minutes for 105° F. The value of
the constant for glue B was about 215 minutes for 70° F. assembly temperature, about 105 for 90° F. , and about 95 minutes for 105° F. If the
pressure was applied so late that the total interval between mixing and
pressing exceeded these times, the joint strength fell off rapidly. These
values have been plotted in figure 3.
Since the interval between mixing the glue and pressing the joints must
not exceed a maximum value for a particular resorcinol glue and gluing
temperature, and since it is equal to the sum of the elapsed times between mixing and spreading and between spreading and pressing, the
longer the glue stands in the pot before it is applied to the wood, the
shorter is the closed assembly time permissible.
Report No. 1546
Phenol-resin glue, intermediate -temperature -setting. --In figure 4 the
average shear test values of the maple block joints prepared with the
phenol-resin glue P in closed assembly were plotted against the age of
the mixed glue at the time the pressure was applied to the joints, as was
done in the case of the resorcinol-resin glues. This figure indicates that
this glue would allow a very long closed assembly
period. No sharp de,
cline in joint strength was noted, and even after 7 days closed assembly
at 70° and .90° F. and 3 days at 105° F. joints were produced that averaged over 2,800 pounds per square inch. At 105° F. the fresh glue gave
consistently higher joint strengths than the same glue after 30 and 45
minutes in the pot.
Melamine-resin glue. --In figure 5 the average shear test values of the
maple block joints prepared with the melamine-resin glue M in closed
assembly were plotted against the age of the mixed glue at the time pressure was applied to the joints as was done in the case of the resorcinoland phenol-resin glues.
The data indicate that at 70° F. , unlike the. resorcinol-resin glues, the
maximum allowable time between mixing the glue and applying pressure
to the joint decreased somewhat as the glue aged in the pot before spreading, from about 205 minutes for freshly mixed glue to 175 minutes for
glue within 25 percent of the end of its working life. This effect of the
age of the glue in the pot was not apparent at 90° and 105° F. The maximum elapsed time (choosing the most conservative value at the 70° F.
temperature) has been plotted in figure 3 as a function of the assembly
temperature.
Open Assembly
Resorcinol-resin glues. --The strength data obtained from shear tests of
joints made with the resorcinol-resin glues in open assembly have been
plotted in figures 6 and 7. From the data it appears that the maximum
allowable open assembly time, with which glue A produced joints on
maple blocks with a shear strength of at least 2,800 pounds per square,
inch, was between 20 and 25 minutes at 70° F. , and between 10 and 15
minutes at 90° F. and 105° F. For glue B the maximum allowable open
assembly times were between 12 and 15 minutes at 70° F. , between 9
and 12 minutes at 90° F. , and between 6 and 9 minutes at 105° F.
Up to 75 percent of the working life, the age of the glue in the pot at
which the glue was spread, seemed to have little or no influence on the
maximum permissible open assembly time for these resorcinol-resin
glues.
Report No. 1546
-5-
Phenol-resin glue. --The average strength data obtained from shear tests
of joints made with the phenol-resin glue P, in open assembly have been
plotted in figure 8. In marked contrast to the case for the resorcinol
glues, the data of figure 8 show the age of this phenol glue at the time of
spreading was very important in determining the maximum allowable
open assembly time. For example, at 70° F. , this glue freshly mixed
produced maple block joints with shear strengths of 2,800 pounds per
square inch with an open assembly period of over 8 hours, but as the glue
stood in the pot, the period of open assembly at which joints of this
strength were made became progressively shorter and was less than 2
hours when the glue had reached 75 percent of its working life at the time
of spreading. For all temperatures and open assembly periods tried
with this glue, it was permissible to apply the pressure at least as long
as the glue in the pot remained spreadable.
Melamine-resin glue. --The average strength data obtained from shear
tests of joints made with the melamine-resin glue M in open assembly
have been plotted in figure 9. Except possibly at 70° F. near the end of
the working life, the data of this figure indicate that the age of the glue in
the pot at the time of spreading had little effect on the maximum permissible open assembly time for this acid-catalyzed intermediatetemperature-setting melamine-resin glue. In all cases the pressure had
to be applied appreciably before the glue in the pot reached the end of its
working life.
Conclusions
For the single-surface spreading, species of wood, curing temperatures,
and assembly conditions employed in these experiments, the following
conclusions seem valid.
Resorcinol-Resin Glues A and B,
Room-Temperature -Setting
The two commercial resorcinol glues varied considerably in the maximum length of assembly time, closed and open, with which strong
maple block joints could be made.
When the resorcinol glues were used in closed assembly, the maximum
allowable closed assembly time was shorter the longer the mixed glue
had been standing in the pot at the time it was spread, and it was
Report No. 1546
-6-
necessary to apply the pressure to the joints within a certain time after
the glue was mixed irrespective of the length of the closed assembly
period. The maximum allowable closed assembly time was at least as
long as the working life (fig. 1 to 3).
The age of the mixed glue in the pot at the time the glue was spread had
no effect on the maximum allowable open assembly time for these resorcinol glues. The maximum allowable open assembly time was always
much shorter than the working life of the glue (figs. 6 and 7).
Phenol-Resin Glue P, IntermediateTemperature -Setting
With the glue of this type selected for study, strong joints were made
after several days of closed assembly time. The age of the glue in the
pot at the time of spreading was more important at 105° F. than at 70°
and 90° F. (fig. 4).
When this phenol glue was employed in open assembly, the age of the glue
in the pot since mixing had an important effect on the maximum allowable
open assembly time (fig. 8). The older the glue mixture the shorter was
the open assembly time at which strong joints could be made.
Melamine -Resin M, Acid-Catalyzed,
Intermediate -Temperature -Setting
For this melamine glue employed in closed assembly, the age of the
mixed glue in the pot at time of spreading was important at the assembly
temperature of 70° F. , but negligible at 90° and 105° F. (fig. 5). In
open assembly, the effect was negligible except possibly at 70° F. near
the end of the working life (fig. 9).
It is important to call attention to the fact that the glues in this study
were spread on only one of the two surfaces to be joined. Double spreading might have given quite different results.
Also, the air movement over the surface of the spread glue during open
assembly has an important effect on the maximum allowable open assembly time. In this study the glue was applied to the surfaces to be glued
in a room in which the air was being circulated by fans, but the surfaces
were shielded from any strong direct draft.
Report No. 1546
-7-
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Figure 1.--Average shear strengths of-joints in maple blocks spread with
resorcinol-resin glue A at three different times during the working
life of the glue and put under pressure after various periods of
closed assembly at 70°, 90° 9 and 105° F.
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ASSEMBLY TEMPERATINE, 70. F
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OF SPREADING
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6—,f1 105 MINUTES
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90
95
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ASSEMBLY TEMPERATUR4 105. F
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AGE OF SLUE IN POT AT TN(
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I
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70
I
75
80
85
90
95
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AGE OF GLLE IN POT WHEN PRESSURE WAS APPL/ED TO THE JOINT (MANES)
Figure 2.--Average shear strengths of joints in maple blocks spread with
resorcinol-resin glue B at three different times during the working
life of the glue and put under pressure after various periods of
closed assembly at 70°, 90°, and 105° F.
17'I 68781 F
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RESORCINOL-RESIN GLUE
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MELAMINE-RESIN SLUE Vs
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ASSEMBLY TEMPERATURE I•P1
IIO
Figure 3.--Maximum elapsed time, between mixing glue and applying pressure, within which strong joints in hard maple blocks were made with
three resin glues in closed assembly.
Z M 68782 F
2,400
3,600
ASSEMBLY TEMPERATURE, 70 • F.
WORKING LIFE, 300 MINUTES
3,400
3,200
4000
2,800 AGE OF GLUE /N POT AT TIME
OF SPREADING
07-0 5 MINUTES
s— —6 150 41/1VUTES
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225 MINUTES
2600
ASSEMBLY TEMPERATURE, 90 • F.
WORKING LIFE, 120 MINUTES
AGE OF GLUE IN POT AT TIME
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0-0 5 MINUTES
h— —A 60 MINUTES
•
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ASSEMBLY TEMPERATURE, 14)5•F.
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2,800
AGE OF GLUE IN POT AT TIME
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Li— --A 30 MINUTES
0---O 45 MINUTES
—
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1
I
2
3
40E OF GLUE IN POT WHEN PRESSURE
7
(DAYS)
Figure 4.--Average shear strengths of joints in maple blocks spread with
phenol-resin glue P at three different times during the working life
of the glue and put under pressure after various periods of closed
assembly at 70°, 90°, and 105° F.
2M 68783 F
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TEMPERATURE; 70•F.
WORKING LIFE, /60 MINUTES
ASSEMBLY
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65
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AGE OF GLUE IN POT WHEN PRESSURE WAS APPLIED TO THE JOINT MI/MUTER
Figure 5.--Average shear strengths of joints in maple blocks spread with
melamine-resin glue M at three different times during the working life
of the glue and put under pressure after various periods of closed
assembly at 70°, 90°, and 105° F.
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GLUE APPLIED
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HAD ELAPSED
HAD ELAPSED
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OPEN-ASSEMBLY TIME (MINUTES)
Figure 6.--Average shear strengths of joints in maple blocks made at
different room temperatures with resorcinol-resin glue A applied at
three different times during the working life of the glue and pressed
after various periods of open assembly at temperatures of 70°, 90°,
and 105° F. Working life of the glue was as follows: upper curves, '
300 minutes; middle curves, 105 minutes; and bottom curves, 60
minutes.
ZM 68785 F
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RESORCINOL- RESIN GLUE
GLUE APPLIED
5 MINUTES AFTER
MIXING
GLUE APPLIED
WHEN 50 PERCENT
OF WORKING LIFE
HAD ELAPSED
GLUE APPLIED
WHEN TS PERCENT
OF WORKING LIFE
MAD ELAPSED
3,400
5,200
4000
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AL
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OPEN ASSEMBLY TIME
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OPEN ASSEMBLY TIME ( MINUTES 1
TEMPERATURE 90 • F
3,600
3,400
4200
3,000
2,800
t,6000
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6 9
0
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OPEN ASSEMBLY nit MINUTES 1
6
9
TEMPERATURE 105 • F
Figure 7.--Average shear strengths of joints in maple blocks made at
different room temperatures with resorcinol-resin glue B applied at
three different times during the working life of the glue and
pressed after various periods of open assembly at 70°, 90°, and
105° F. Working life of the glue was as follows: upper curves,
210 minutes; middle curves, 90 minutes; and. bottom curves, 60
minutes.
Z K 687AR F
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PHENOL - RESIN CLUE
GLUE APPLIED
5 MINUTES
AFTER MAKING
RPM
GLUE APPLIED
WHEN 50 PERCENT
OF WORKING LIFE
HAD ELAPSED
GLUE APPLIED
WHEN TS PERCENT
OK WORKING LIFE
HAD ELAPSED
4400
3,200
4000
2,800
• 2,600
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OPEN ASSEMBLY TIME MINUTES
TEMPERATURE 70° F
3,400
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3.200
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3 DO 360 420 400
50
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80
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20
30
40
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OPEN ASSEMBLY TIME (MINUTES I
TEMPERATURE 90 • F
5
4400
o►
.X200
3,000
2,800
2,600
90 90 /00 /10 30 40 50 60 10 20" 30 40 SO
OPEN ASSEMBLY TIME (MINUTES)
TEMPERATURE 105° F
Figure 8.--Average shear strengths of joints in maple blocks made at
different room temperatures with phenol-resin glue P applied at three
different times during the working life of the glue, and pressed
after various periods of open assembly at 70°, 90°, and 105° F.
Curing temperature was 180° F. Working life of the glue was as
follows: upper curves, 300 minutes; middle curves, 120 minutes; and
bottom curves, 60 minutes.
2 M 887877
MELAMINE — RESIN GLUE NM"
GLUE APPLIED
GLUE APPLIED
GLUE APPLIED
5 MINUTES
WHEN 75 PERCENT
WHEN 50 PERCENT
OF WORKING LIFE
OF WORKING LIFE
AFTER MIXING
HAD ELAPSED
HAD ELAPSED
moas,
3,400
3,200
3,000
2,900
2,600
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OPEN ASSEMBLY TIME ( MINUTES )
TEMPERATURE 70 • F
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3,
3.400
lat
4200
o 4000
416 Asoo
o
5
/0 al 0
5
A,
IS 0
5
/0 /5 20
OPEN ASSEMBLY TIME MINUTES I
TEMPERATURE 90 • F
los
5,400
3,000
AL
MENMINNAMII
2,200
El
3,200
2,600
EMIIII 1111
0 3 6 9 0 3 6 9 0 3
OPEN ASSEMBLY TIME ( MINUTES )
TEMPERATURE /05• F
6
9
Figure 9.--Average shear strength of joints in maple blocks made at
different room temperatures with melamine-resin glue M applied at
three different times during the working life of the glue and
pressed after various periods , of open assembly time at 70°, 90°,
and ' 105° F. The curing temperature was 140° F. Working life of
the glue was as follows: upper curves, 160 minutes; middle
curves, 80 minutes; bottom curves, 60 minutes.
1
m 68788 F
It