EFFECTS CIF MOISTURE ON
EACTERIAIL WEAKENING CIF
CASEIN-BONDED PLYWOOD
No. 2077
April 1957
INFORMATION REVIEWED
AND REAFFIRMED
1963
PO FILE COPY
113 1:31 REMOVE FROM FILE
Ii111
131111
IIIIIIIIIIIIII!111!1111! IIMIL
a'
FOREST PRODUCTS LABORATORY
MADISON 5. WISCONSIN UNITED STATES DEPARTMENT OF AGRICULTURE
FOREST SERVICE
fn Cooperation with the University of Wisconsin
EFFECTS OF MOISTURE ON BACTERIAL
WEAKENING OF CASEIN-BONDED PLYWOOD
By
CATHERINE G. DUNCAN, Pathologist
Forest Products Laboratory„ 1- Forest Service
U. S. Department of Agriculture
Summary
Shear tests were made on standard specimens of 3/16-inch, casein-bonded plywood to determine the effects of bacterial infection of the glue line with
the wood at various moisture content levels. The results indicated that within 31 days the bacteria, all of which were capable of digesting casein, did
not materially weaken the glue joints when the moisture content of the wood
was below about 55 percent. Above 55 percent, however, the bacteria caused
considerably more weakening of the joints than could be accounted for by
hydrolysis alone. Slight bacterial deterioration of the glue was indicated
within 48 hours, indicating that bacteria may become active in a very short
time if the moisture content of the wood is high enough. The question
remains whether molds may also weaken glue bond.
Introduction
Previous experience and laboratory tests have shown that the bond of caseinglued plywood may weaken and ultimately fail when exposed continuously to
warm, humid conditions. This failure has been variously attributed to chemical hydrolysis, biological deterioration of the protein material in the glue,
or mechanical stresses induced by shrinking and swelling.
Christensen and Moses a showed that naturally occurring infections of molds
and bacteria play an important role in the deterioration of casein glue, at
least when the moisture content of the wood is above 50 percent. Their
Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
-Christensen,
-Christensen, C. M., and Moses, C. S. Molds and bacteria that delaminate
plywood bonded with casein and soybean glues. For. Path. Spec. Release
No. 25. 1945.
Rept. No. 2077
-1-
Agriculture-Madison
findings, however, did not fully establish whether there may be weakening of
the glue bond because of hydrolysis alone, or whether both molds and bacteria
can deteriorate the bond.
The present study was undertaken to ascertain what effect different amounts of
moisture in casein-glued plywood may have on the rate of bacterial deterioration of the bond and how much deterioration may be expected from the same
amounts of moisture in the absence of any micro-organisms.
Materials and Methods of Test
Longitudinally matched sets of 3-ply casein-glue bonded panels were made from
rotary-cut, yellow birch veneer 1/16 inch thick. After conditioning at 80° F.
and 65 percent relative humidity for 3 weeks, the panels were cut into standard 1- by 3-1/4-inch shear-test specimens with the grain of the face plies
running lengthwise. (The two standard saw cuts in the face plies were made
just before the specimens were tested for shear strength.) The specimens
were then returned to the conditioning room for 6 months before being tested.
Specimens for each test variable were selected at random.
The test variables considered were as follows:
Specimens at various levels of moisture content incubated singly in closed
bottles for 31 days at 93° F. (34° C.).
Group
A. -
Specimens with 8 percent moisture content, sterilized, not inoculated.
Group 132 - Specimens as in Group A except not sterilized.
- Specimens soaked in distilled water under partial vacuum, dried to
Group
different levels of moisture content from 30 to 100 percent,
placed singly in the incubation bottles, and finally sterilized to
free them of infection.
Group D - Specimens as in Group C except that, following sterilization, they
were aseptically inoculated with a mixed suspension of the five
selected bacteria.
Specimens wetted and incubated simultaneously by soaking in distilled water
for 48 hours.
Group El - Specimens and soak water sterilized.
.Group F - Specimens as in Group E except that, following sterilization, they
were inoculated by incorporating a mixed suspension of the five
selected bacteria in the soak water.
5-Groups in which the action of micro-organisms was prevented by the dryness of
the wood or by sterilization.
Rept. No. 2077
-2-
Group G - Neither specimens nor soak water sterilized or inoculated. Thus,
any such infection occurred largely through natural infection of
the glue.
The tests were made in screw-top glass jars 2 inches in cross section and
4-7/8 inches high at the shoulder. To prevent unequal absorption of any free
moisture that might condense on the walls of the jars during sterilization,
the specimens were suspended from a wire soldered to the metal lids. Moisture
losses during testing were prevented by sealing the lids to the jars with a
mixture of warm paraffin and beeswax.
An attempt was made to bring the moisture content levels of the specimens to
points ranging between 30 and 100 percent. To get a reasonably uniform distribution of moisture within each specimen, the conditioned samples were
first placed in distilled water under a reduced pressure of 6 centimeters of
mercury for 1 hour. They were then distributed in desiccators and dried for
periods varying from 30 minutes to 4 hours in order to bring the moisture content of the specimens to varying moisture content levels within the desired
range of moisture content.
The specimens to be sterilized were autoclaved for 30 minutes at 15 pounds of
steam pressure on two successive days. Preliminary tests had indicated that
all micro-organisms in the glue would be killed by this procedure.
The inoculum consisted of a mixed culture of five strains of bacteria selected
from among those isolated when dilutions of several manufactured casein glues
were made and planted on tryptone-sodium-caseinate-glucose agar and a 2 percent trypticase agar adjusted to a pH of 7 to 8. Those selected had previously been found independently able to delaminate plywood bonded with a
casein glue.
After the specimens were sterilized and cooled, those to be inoculated were
dipped for 3 seconds in a suspension of the 5 strains of bacteria; the controls were similarly dipped in sterile distilled water. Care was taken in
both instances to avoid any contamination. The material, except that used in
the initial dry and 48-hour soak tests, was incubated for 31 days at 93° F.
(34° C.).
Shear strength values were obtained for all specimens in each test category.
Attempts were also made to isolate from the glue lines bacteria used to inoculate the specimens or any contaminants that might have been introduced. To do
this, the specimens were first flamed to eliminate surface contaminants. The
plies were separated with a flamed knife, and scrapings of the glue from the
center and edge were placed on plates containing trypticase and malt agars
and in tryptone-glucose broth. They were then incubated for 14 days at 93° F.
(34° C.) under both aerobic and anaerobic conditions.
Rept. No. 2077
-3-
Results and Conclusions
The shear strength and final moisture content of each specimen following incubation are shown in tables 1, 2, and 3, and average values for the 31-day
incubation test are shown in figure 1.
The findings and conclusions may be summarized as follows:
1. Comparisons (table 1) of test groups A and B, and of groups G and E, show
that sterilization in itself had no sizable effect on the strength of either
the dry specimens or those soaked for 48 hours.
2. Comparison (table 1) of the strength of the nonsterile specimens (groups
G and F) with those of the sterile specimens (group E) gives some indication
that bacteria may have caused small deterioration of the glue during the
48-hour soak.
3. Consideration of specimen group C in figure 1 discloses that, in the
absence of active micro-organisms, there was a progressive reduction in
strength during the 31-day storage period as the moisture content increased
from 8 to approximately 55 percent. Further, but much smaller, reductions in
strength occurred as the moisture of the specimens was increased from 55 to
approximately 100 percent. The maximum reductions left the joint strength of
the specimens approximately one-third as large as the presumed original
strength of the dry specimens of groups A and B.
Attempts to make isolations from these specimens indicated that all bacteria,
yeasts, and molds had been killed by the sterilizing process and that no
later contaminations by micro-organisms had occurred. Therefore, the losses
in strength must have been due to hydrolysis rather than to biological deterioration of the glue.
4. The sterilized and subsequently inoculated specimens incubated 31 days
(group D) showed joint strength values only slightly lower than those of
the corresponding uninoculated specimens (group C) when moisture content
levels of the wood ranged between 35 and 55 percent. Above 55 percent moisture content, however, the joint strength of the inoculated specimens was for
the most part less than half of that of the uninoculated specimens (fig. 1).
A large share of this difference occurred in the approximate moisture range
of 55 to 65 percent. Bacteria were isolated from the glue line of all the
inoculated specimens with a moisture content of 55 percent or more. Because
the only difference between the inoculated (group D) and uninoculated (group
C) specimens was the presence of living bacteria, it may reasonably be concluded that the smaller shear strength values of the inoculated specimens in
the moisture range above about 55 percent were the result of bacterial action
on the glue lines.
Rept. No. 2077
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Table 3.--Shear strength of casein-bonded plywood specimens tested after 31 days at various levels of moisture contentl.
Panel :
Specimens sterilized, inoculated! (Group D)
: Moisture I Strength/ : Moisture : Strength/ : Moisture : Strength./ : Moisture : Strength/ : Moisture : Stre : content :
: content :
: content :
: content :
: content :
: Percent :
A
37
:
:
: Percent :
260-0
47
:
180-0
:
49
:
185-0
54
:
:
:
48 195-0
55
;
59
: 150-o
57
2
:
:
: Percent :
:
!
.
:
1
31
: 260-0
37
: 255-0
36
!
:
:
250-0
: Percent ,
:
1
155-0
: 60-0
:
: 70 - 0
68
2
160-0
:
130-0
61
:
56
:
140 - 0
:
66
: 100 - 0
47
: 280-o
;
53
g
150 - 0 61
!
:
53
155-0
5-a
155-o :
t
:
53
180-o
73
:
:
90 - 0
:
.
: 65-0
:
:
:
:I
:
:
: :
58
!
•
c
:
:
170-0
r
180-0
.
34
1
265-o:
45
34
;
280-0
;
46
32
;
265-0
•
32
;
32 : 285 - 0
:
: 240 - 0
295-0
:
66
; 80 - 0
185 - 0 64
:
51
170-0
:
62
: 65-0
:
: 170-0 :
65
:
.
250-0 57
50
:
58
:
:
180-0
72
r
: 45 -0
% 101
: 45-0
71
:
80 - 0
;
; 40 - 0
70 70-0
7
r
55
.
;
•
•
g
180-0
:
:
:
:
114: 40 - 0
:
56
:
:
1
10665-o
:
111
i 60 - 0
:
170 - 0
.:.
5'
:
2
1
:
70- 0
: 45 - 0
.
195-0
7
: 111
:
!
:
: 45-0
69 95-o
: 60-c, 61
44
7 :
:
62
: Percent :
11
1
:
115-0
57
1 '2
: Percent :
155-0
145-0
54
:
52
: Moisture : Strength/
: content :
:
90-0
g
: 45-o
65 80-o
104
:
7
:
I,
•
D
185-0
:
55
:
48
185-5
.
56
. 185-0
54
56
51
240 - 10
:
r
1
:
.
t
:
:
I
•
, 80-0
:
65
60-0
:
170 - 0
62
85-o ;
190-0
62
:
175-0
:
34
1
295 -0 1
45
0
:
1
:
g
:
56
145-0
1
;
ah
,-
:
r
!
:
59
125-0
;
:
53
:
: 268-o
7
:
46
:
224.1
:
:
!
160-0
:
,
145 - 0
135-
:
:
!
105
:
50 -0
: :
:
:
1
2
:
110.0
I
120-0
!
55
:
160-0
;
:
:
:
:
i
; 7o - o
:
7
61
: 75-0
!
70
7o
!
1
:
8
.
:
:
s
:
I
80-0
:
: 109
40-0
95-0
103
35-0
:
85-0
:
:
.
:
:
1
:
1
f:
:
7
!
66
1 57
!
;
:
:
240-0
•
34
185-0
i
:
si
57
245 - 0
r
:
:
:
1
:
250-0
49
t
1
54
85-5
:
:
:
1
1 :
2
51
44
Average:
64
•
.
:
:
;
E
195-0
47 43
190-0
!
:
53 2
64
. 74-0
.
: 74-0
72
c
: 108
: 49-0
APercent moisture (based on ovendry weights) at time specimens were broken.
2 1t can be assumed from representative culturing of the glue lines that bacteria were present in all unsterilized specimens and in sterilized
and subsequently inoculated specimens. There was no evidence of living bacteria in sterilized, uninoculated specimens.
/The first value represents the shear strength in pounds per square inch; the second value represents the percentage of wood failure. Each
value is for one specimen, except where averages are given.
Sept. No. 2077