U. S. Department of Agriculture, Forest Service
FOREST PRODUCTS LABORATOR Y
In cooperation with the University of Wisconsin
TESTS TO DETERMINE THE TIME REQUIRED FO R
SILICATE OF SODA TO REACH ITS MAXIMU M
HOLDING POWER WHEN USED ON FIBREBOAR D
o-
By H . M . HAL E
Industrial Expert in Forest Products
.
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TESTS TO DETERMINE THE TIME REQUIRED FOR SILICATE OF SODA TO REAC H
ITS
?I,M%l SMOG POWER WHEN USED 0 t$RE BOARD
Introduction
. Following a series of e cperimeni_s ,on 150 fibre board boxe s
containing L one-pound condensed milk cams,• in which it was observe d
that 14 percent of the failures were trasea b ,l.'- e to failure at the fla p
joint when the box was sealed, the experients ineluded in this repor t
were made .
The details of the previous experiments are to be found in th e
First Progress Report, "Tests on Fibre Board Shipping Containers for th e
Improvement of Design and Specifications," by A . H . Mackenzie, approve d
December 30, 191 g .
The high percentage of sealing joint failures with the us e
el silicate of soda indicated the need for the de :ve,lopment of a sealing
method which would redure the percentage of joint failures before othe r
methods of improving the design could be takpm up .
The earlier teats suggested five possible so zrces of weaknes s
at the joint :
1 . The length of time required for the si irc 'e at' soda to set, and
the wet paper to regain its strength .
2. The composition and specific gravity of the silicate off` soda.
3. The temperature conditions under which the seal
4. Lack of sufficient pressure to insure coit .ac*
..
5. The temperature Q.f the box after aaaling .
8376
is done .,
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1 ti
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. - 1 '_ The first condition appears to be the controlling one ; the
*econd and third are negligible under ordinary conditions ; the fourth i s
important to the degree of insuring contact only ; the fifth has some effect ,
but just how much is difficult to determine .
i
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ishould be noted that in the second condition but tw o
1of silicate of soda were observed and the chemical analysi s
teem to be very similar . The samples represented each c1 .ss .
Pressure sufficient to insure contact is necessary . qge t
unit**
ults . In tests on fibre boxes it was found that the sealin g
was
pendable when the covers were weighted with approximately 5 0
en with this weight carefully applied it is unusual to obtai n
t'C'0s1t tf joint contact between the flaps . The warp in the tw o
beards coax osing the flaps is sufficient to hold them apart for at leas t
i ,• .
- A
d doubtle s has an
The
ratura
'
a bl+1
ill ~!! 44re
S
.0~e . This is a deductio n
sis of the very short time required fo r
Bch can safely be made on t'
t* silicate tg_ ;.et Q, the emples at a temperature . of 120° F .
' Methods
ter '
• "
:--
'$ections of fibre 1o`aA'.' approximateTel inch by - 1/2 inche s
1/2 inches and joined with silicate of soda . The sample s
owed to dry for periods varying from 5 minutes to 20 hours .
2Dg,-Lilaw intervals of drying from 5 minutes to 20 hours were observed .
040 ■ak Or samples was dried under pressure of 2/3 pounds per square inc h
aAOWNPt . The other series was dried without continuous pressure . The
0 066
which were .O SO inch board rated at 200-pound Mullen test ,
the Reihle cement testing machine . The board unde r
* brea at 290 pounds per 1 inch cross section . Similar sample s
-OW hilted aft silicate applied to (1) one board, (2) to both boards .
uelzr
were
Resullwhich are shown in ,elMjg'~'-- in the attached tabulatio n
than
11% That
,
e applied ' a- lob
duces no stronger join
VS 'loth s •aces
frequeatly
re4lts
in a weaker one .
•
2 . Th4 1 ilicate of soda addiFieflWiNlithat no failures occur i n
t Sint
to the silicate
ly aft* 10 minutes and very seldom
at 5 minutes . The joint does not appear to get its maximum strength fo r
about 4 hours, probably due to a softening action in the kraft itself o r
in kraft chip glue .
3 . The application of pressure to the glued joints usually result s
in a slightly higher average breaking strength than when no pressure i s
applied . A careful analysis of the original sheets, however, shows tha t
the individual samples break at a much more uniform load when they hav e
been dried under pressure than otherwise ; in other words, the extremes ar e
narrower .
The conclusion is that the maximum strength is not reache d
until 4 hours have been given the silicate of soda for setting. Th e
intermediate findings between 5 minutes and 4 hours do not follow a
regular curve, either for the samples under pressure or no pressure . At
5 minutes the breaking strength is 55 pounds with one sample of silicat e
of soda and 54 with the other . This indicates no difference in th e
strength of the two samples of silicate of soda . At 4 hours one sampl e
shows a breaking strength of 109 pounds and the other 106 . Th e
inconsistent part of the results is found in the intermediate periods .
For example, the tests with one sample show the breaking strength a t
30 minutes to be greater than at 40 minutes, 60 minutes, 1-1/2 hours o r
2 hours . At 4 hours the strength equals that at 30 minutes . Probably a
softening action has taken place in the kraft itself or the kraft-chi p
glue, which is not overcome until 4 hours or more have elapsed . One
point is clear, namely that maximum strength at the joint is seldo m
reached in less than 4 hours . After 30 minutes or more setting th e
silicate joint shows an average strength of between 80 percent and 9 0
percent of the maximum strength .
It has been determined that the application of the silicate o f
soda to the fibre board softens the kraft liner, which is the larg e
factor in the tensile strength of the board . This absorption of th e
water from the silicate of soda weakens the joint by this softening
action .
A possible explanation of the erratic results as indicated o n
the accompanying chart may be found in the fact that the silicate of sod a
was applied with a brush which makes the absolute control of the quantit y
applied impossible .
The absorption of moisture tends to weaken the silicate join t
between the kraft and the chip . Before the board can reach its maximu m
strength, this water must have been evaporated through it, which becaus e
of its thickness is a slow process . This accounts for the fact tha t
several hours are necessary for the joint to reach its maximum strength .
All failures occur in the joint by the kraft splitting or pulling fro m
the chips .
x376
-3-
-1
'tf soda, one obtained in th e
,41 ,c
e► it market 06oc.al drug OiWeo, the second from a large manufacturer o f
bested in 4etopacison to determine their relative strengt h
fibre boxeCy'w.
ode as the result of laborator y
in a join$ This iii a experir mte w
ai j oint failure s
record?s s-h 1
anti exe
Tonally Mgt' pe!x=
a
.' ' ,4laO
in a .series, 'bf drum tests on fd
texas . .
..
kv . The 140 00 two batc :Mk'
st revealed
An analysis of these samples by the :laborat p
loo d-- silicat e
no marked difference in their composition . The two s•
of soda were carried through a series of identical t€ ts, aid alined i n
lusivelo That the
cif this report . Results show c
the first pap ii silicate of soda purchased in the open market ways .izliy as ERo% if no t
better, than the article furnished by the box mere, %el teaure g' noted i n
earlier drum tests can safely be ascribed to in st fi€ -exit tip* having bee n
wstse/m4o eW 6e4m to set . No record was n !e of Wt *time allowe d
Strum
_
for Petting in the earlier experiments .
Vib sQialysis of the silicate of soda sainp&e
OW4 is as follows :
. Per- :Percent :Silicate :
Sample
:Specific : Per- : cent : silica : n- umber :Original : Organi c
number and :gravity : cent : total : T .S . - :% silica :
: matte r
description :
: Na 20 :solids : Na20
: %Na20 .
No . 1 from
:
C . M .& L . Co . : 1 .3918 : 8 .1+5 : 37 .17 : 28 .72 : 3 .40
No . 2 from
dru- stare
:
:
. 1 .3979 : 8 .72 : 141 .27 : 32 .55 : 3 .73
:Colorless
. Non e
.
:Caramel : Consider abl e
.
Effect of Temperature on Rate of Drying ,
e.Testmado rinethfcoempratunh
strength of the silicate joint at varying periods of time after joining s
were made on sample strips of fibre board . The results show that fo r
normal working temperature, that is, between 450 F . and 760 F ., the
influence is so slight that this factor could not be detected .
That temperature has a marked effect on the rate of drying unde r
extreme conditions was demonstrated .
8376
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At 120° F. the joint reached W percent of its maxis= strength
in 2 minutes . The average maximum strength -circler these temperatur e
conditions was reached in 5 minutes . Humidity was not determined, but as
the work was done in an ordinary electric drying oven the humidity w
not be excessive . At 32° F . more than 3 hours were required to reac h
approximately the same result .
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TESTS ON SODIUM SILICAT E
Box Mfgr . Sample 2 Nat S103
Working Temperature 68° F .
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Time
1
5 min .
10
II
20
II
30
,I
40
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6o
Ir
90
n
120
tl
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11
72
8
43
10
64
6
53
9
8
10 9
°
1O
83
6o
10
82
10
5s
10
85
10
56
_19
5a
9
91
.-.. J
20
C
10 8
2
17
81
16
90
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Pressurai
No pressur e
Pressure 2/3 lbs . per sq. in,
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54
8
~.~rtasa
240 I I i7I
.1.i~.1T~r'rr/
20 hr .
10
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40
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Time
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60
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120 s
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240 n
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40 R
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g5
61
10
g
11
59
9
10
93
39
10
89
5
7s
10
86
10
67
0
10
73
52
10
72
75
"T .77 71
AP71r
10 5
S
D
2
L
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Pressure 1
I No pre s sure; 7
Pressure 2/3 lbs . per sq. in.
55
56
10
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24
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9
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30
a
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5 =in.
10
+,
TESTS ON SODIUM SILICATE
Drug Store Purchase Sample 2 N A2 51 03
Working Temperature 68° F .
2
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12 0
83