` AGRICULTURE ROOM
U. S. Department of Agriculture, E?rest Service
FOREST PRODUCTS LABORATOR Y
In cooperation with the University of Wisconsin
MADISON, WISCONSI N
Ot te'-k
MINIMIZING WOOD SHRINKAGE AND SWELLING
Effect of Heating in Various Gases
By ALFRED J . STAM M
Senior Chemis t
and
L . A. HANSEN
Former Senior Scientific Ai d
4RA
AUG
1
r
~~ srATE
0HOO . OF FORESTR Y
OREGON STATE COLLEG E
CORVALLIS . OREGON
Published i n
INDUSTRIAL & ENGINEERING CHEMISTR Y
July 1937
195 8
,
4
MI TrMIEING WOOD SHRINKAGE' AIM ~-AIN G
Effect of Heating in VariO.us: Gases'
i r=
By
A . J . STAM, Senior Chemis t
and
L . A . HANSEN, Former Senior Scientific Ai d
.f
Abstrac t
J
The hygroscopicity and subsequent swelling and shrink ing of dry wood is decreased by heating in various gase s
above thermal decomposition temperatures . Greater reductions in hygroscopicity are obtained in an oxidizing than in a re ducing atmosphere for the same heating conditions, but by in creasing the temperature equal reductions in hygroscopicit y
can be obtained in reducing atmospheres . The darkening o f
the wood on heating appears to vary directly with the re ma t ing reduction in hygroscopicity, regardless of heating c- ditions . Snaking in water after heating has but a slight tend ency to restore the original hygroscopicity . Heating wood i n
water-saturated atmospheres has no permanent effect upon th e
swelling and shrinking .
It has long been recognized that excessive heating of woo d
reduces its hygroscopicity . Tiemann (9) found that heating air-dr y
wood in superheated steam to about 150° C . for 4 hours reduced the sub sequent moisture absorption by 10 to 25 percent with but relatively smal l
reductions of the strength except for red oak which s :lke+w•ed a reductio n
in crushing strength and modulus of rupture of about 60 percent . An unpublished Forest Products Laboratory report of 1916? shows thO heating
black gum in dry air at 205° C . for 6 hours reduced the subsequent hygro a .Ligh t
scopicity to almost one-half of its original value with on:
accompanying dcrvase in its strength . Koehler and Pillow ) as-a Pillow
1
-Presented beffiire tFie: Cellulose Division, ,n rican 0)b*
Chapel Hill, N . C ., Apr . 12, 1937 .
-Betts, N . D . The Effect of High Temperature on Cent,-ice- rx o
Wood . File No . 2B46&, Forest Products Laboratory .
•
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2,
,,.
1
'
air-slr. Sitka spruce and ash to 13 ° C . for 1 toe days an d
of the equilibrium moisture content at ievera l
-es for the longer time of heatino of 30 t o
ring reductions of the crushing strength of 1 5
ceo
keductions of the toughness of 50 to 75 percent .
Data of i
, ) for the relationship between maximum ttrength o f
beech in T7.Rsion perpe lent tz no rain and the moisture conten t
at different temperatures indicate that the loss of strength on heatin g
of the wood decrea s .w tl ,a. decrease in me.l.p4tw.e cant-ent to a neglid,
gibe vase
very
b7 Worm
be1o
emp
g
rti
_~
I
strength occurring i f
tem
to1 0
d
r
ion.
was heated o
asbestos
lagg
l LWtibi
•le inser
a
insulation . The temperaturelqpp 4
in a well in the wall of the bomb . The temperature was con rolle d
manually to about 2° C . When the specimens were heated in other gase s
than air the bomb was evacuated and the gas admitted several times t o
insure the elAmination of air . The moisture content of the wood use d
is the t
.s about 6 percent, When the heating was done in th e
pw4se;oe or *titer vapor a large excess of water over that necessary t o
i~.xa,te 't~.e',?~ s placed in the bottom of the bomb .
t
min' :
o
a
Y T~I
shri
d~ff~a;~ i e-,y atme'Vheres upon tll;ei o t.
a Slae n t swelling
Neasz
m were .de of both
t
o il. ' ; v : nsion mange an'± 7
vie ,g 4t MjNite 1YC
when the
t ernate lbrought :'fir ns'
dity row .
e .li
with 3Q
Yo l!,‘O,oea rotative humidity in Y
r
e are
held a . g o C.,
we
gkv
.
,
tTga
.
* specim±e $
$
t
! .fir
e :librium .
relative h idity
h prove.
idj ,
Titt antish• nk off 4ilggOgitaitl
e (eductio n
{
in di ms
t
'
cent
relative
e
am
:'t
sec.,
,
t
W
The
ewhfdsi/g
Ao
► qtr tira
.toted in atmospheres were soaked in water for five days afte r
the li4ity cycles were complete and then subjected again to the humidit y
changs . Iles . Only the second subsequent humidity cycle was used in th e
c
.t4ons . the ft. could involve a higher desorption curve due t o
T
.
1
r
A142
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the soaking and thus give results affected by the sorption hysteresi s
(10) . The same is true for sections heated in the presence of water .
The first cycle gave appreciable negative efficiencies because of this ,
hysteresis effect .
Heating the sections for as short a time as 15 minutes a t
C ., a temperature at which thermal decomposition is just becomin g
appreciable, gave definite antishrink efficiencies in all the dry gases ..
Increasing the temperature and the time of heating increased the anti shrink efficiency . In each case the efficiency was greater in an oxidizing atmosphere than in a reducing atmosphere . Subsequent soaking of the :
sections in water reduced the antishrink efficiency by a relativel y
constant amount regardless of the heating conditions . Sections heate d
in the presence of water vapor gave very small uncorrelatable antishrin k
efficiencies after the first cycle, part of the values being positive an d
part negative (average antishrink efficiency 0 .25 percent, mean deviatio n
1 .1 percent) . These values for all the temperatures and times may b e
considered within experimental error of being zero ; that is, heating i n
water vapor has no effect upon the antishrink efficiency after the firs t
humidity change cycle .
165°
The sections heated at 165° C . were but slightly darkene d
without a perceptible difference in appearance between the section s
heated in the different gases . The darkening was more appreciable afte r
heating for 2 hours at 205° C . and still more so after the time was in creased to 6 hours . In each of these cases the darkening obtained b y
heating in the different gases increased in the following order :
Hydrogen, illuminating gas, air, and oxygen . The specimens heated t o
260° C . in hydrogen, however, were as dark as those heated for 6 hour s
at 205° C . in oxygen . The darkest specimens were about the color o f
unfinished walnut . The antishrink efficiency appears to parallel th e
darkening of the wood, irrespective of the temperature and the gas used .
•
Although part of the antishrink efficiency may be due to oxidation in the cases where the wood was heated in air and oxygen, it is har d
to imagine this being a major factor as equal efficiencies can be obtaine d
by heating in hydrogen at a slightly elevated temperature . The phenomeno n
can best be explained on the basis of the effect being one of therma l
decomposition . Loss of water of constitution is the first thermal re action . If this loss were due to the formation of an ether linkage between two adjacent cellulose chains through adjacent hydroxyl groups ,
the loss in hygroscopicity could be readily explained . Not only woul d
the hygroscopicity be reduced because of the substitution of the les s
hygroscopic ether group for the more hygroscopic hydroxyl groups, bu t
also because of the parallel bonding of the cellulose chains . Staudinger
( 8 ) has shown that the formation of such bridges between the chains i n
polystyrene
resins with paradivinylbenzene cuts down the swelling
tremendously, even when only enough paradivinylbenzene is used to for m
a single bridge for several thousand molecules of monomeric styrene . Jus t
an occasional cross link evidently cuts down appreciably the tendency fo r
water to be taken up between the structural, chains, The formation o f
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tther linkages between the hygroscopic hydroxyl group not only explain s
he decreased hygroscopicity of wood heated in dry atmospheres, but als o
he fact that heating in the presence of a large excess of water vapo r
clauses no change in hygroscopicity . The presence of an excess of wate r
laser would depress the thermal reaction in which water is evolve d
according to the principle of LeChatelier and thus markedly reduce th e
tendency- to form the ether bridges . If the change in hygroscopicit y
merely a physical change, such as that postulated by Urquhar t
0)
r e to explain hysteresis, soaking of the specimens in water shoul d
largely restore their original hygroscopicity . This investigator be lieves that the free hydroxyl groups of cellulose, which are originall y
satisfied to a large extent by water, draw closer together on dryin g
•,nd finally mutually satisfy one another . Although these bonds are onl y
partially broken on rehumidification they should be largely broken o n
soaking in water and the active groups again satisfied by water . Th e
reversible part of the antishrink efficiency, that is, the differenc e
between the antishrink efficiency obtained directly after heating an d
that subsequent to soaking in water is practically constant regardles s
of heating conditions . This part is undoubtedly due to a physical effec t
such as that given by Urquhart (10) . The physical mutual satisfactio n
of hydroxyl groups evidently increases until the free water is rathe r
completely removed, but does not increase on further heating .
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These preliminary results indicate that the antishrink efficiency resulting from the excessive heating of dry wood in several commo n
gases is sufficiently great and permanent to warrant a more extensiv e
investigation in which the strength properties are simultaneously studied .
Although this method of minimizing the swelling and shrinking of woo d
does not appear to be so effective as methods previously described (1, 3 ,
6, 1), its possible value rests on the fact that it would be relativel y
inexpensive .
Literature Cite d
4
(1)
(2)
Browne, F . L .
Ind . Eng . Chem . 25 :835 ( 1 933) .
Greenhill, W . L . Jour . Council Sci . Ind . Res .
(Australia)
9 :265
(1936) .
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
Hunt, G . M . Circ . 128, U . S . Dept . Agr .
(1930) .
Koehler, A . and Pillow, M . Y .
South .Lbrman ., p .219, Dec . 19, 1925
Pillow, M . Y . Wood Working Industries, p .8, Oct . 1929 .
Stamm, A . J . and Hansen, L . A .
Ind . Eng . Chem . 27 :l4.80 (1935) .
Stamm, A . J . and Seborg, R . M .
Ind . Eng . Chem . 2$ :1164 (1936) .
Staudinger, H . Trans . Faraday Soc . 32 :323 (1936) .
Tiemann, H . D . Lbr . World Review 25(7) :10 (1915) .
Urquhart, A . R . J . Text . Inst . 23 :125; (1932) .
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Table 1 .--Effect-of heating dry wood in various gases and subsequent soak-
ing in water for5days upon the antishrink efficiency
A
Gas
I
Antishrink efficiency : Antishrink efficienc y
:before soaking in waterl : after soaking in water ?
:
: Tangential
Weight
: Tangential : Weigh t
dimension :
basis
: dimension
basi s
basis
basi s
:
:
Hours :
P e r c e n t
:Temper- : Time
: ature : of
heat
ing
°C .
165
205
260
0 .25 :
: 2 .00 :
: 2 .00 :
5 .9
I6 .0
32 .0
Illuminat- : 165
ing gas
: 205
205
.25 :
: 2 .00 :
: 6 .00
8 .2
18 .0
20 .6
Air
:
.25 :
2 .00 :
: 6 .00
.25
: 2 .00
6 .00
Hydrogen
1xygen
:
: 165
205
205
165
205
205
2 .8
11 .5
31 .8
I .g
:
11 . 14
: 31 . 2
8 .5
19 .0
19 .0
6 .3
114 .0
19 .6
4. s
13 . 2
17 . 6
g ,3
17 .5
23 .2
6 .4
19 .0
21 .2
4 .9
1 14 .0
22 .0
10 .0
20 .7
28 .0
12 .0
21 .0
30 .0
7.0
15)4
28 .7
▪
▪
6 .3
17 .0
32 .0
:'A
4. 4
12 . 3
21 . 0
6. 1
13 . 6
27 . 2
1
-In terms of retardation of the dimension and weight changes, for th e
average of four specimens, per unit change of the untreated control s
when alternately brought to equilibrium with 30 and 90 percent relativ e
humidity .
i
?Based upon the second humidity change cycle as the first is appreciabl y
affected by hysteresis (10) .
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