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SYMPOSIUM (IDS'86)
. . Cambridge, Massachusetts, U.S.A-August 13-16, 1986
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EXPERIMENTAL ANALYSIS OF THE DIFFERENT PHASES LN THE DRYING OF NATURAL RUBBER
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/ * ORSTOM : I n s t i t u t F r a n ç a i s d e Recherche
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Scientifiqu,e pour l e 0 1
Développement en Coopération.
L a b o r a t o i r e del Génie C i v i l , U n i v e r s i t é des Sciences e t Techniques
Place E. B a t a i l l o n ,'34060 Montpellier Cedex, France
du Languedoc
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1.INTRODUCTION
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Latex has t h e a s p e c t of a rubber p a r t i c l e
d i s p e r s i o n i n a water serum containing o r g a n i c and i
m i n e r a l elements i n s o l u t i o n
I t s content i n
..
s o l i d matter i a approximately 30 t o 40%
90% of
which being formed of rubber p a r t i c l e s , The
r e m a i n d e r . i s made up of non-rubber elements
(proteins,sugars,etc;.)
[I). The composition of
:
l a t e x i s complex and prone t o changes l i n k e d t o
v a r i o u s parameters : c l o n a l c h a r a c t e r i s t i c s ,
c l i m a t i c i n f l u e n c e , e t c ; . From t h e l a t e x c o l l e c t i o n
on p l a n t a t i o n t o t h e i n d u s t r i a l use under i t s
v a r i o u s forms, numerous process a r e involved t o
o b t a i n a f i n i s h e d product. Drying is one of t h e s e
i n t e r m e d i a t e processes ; i t i s a d e l i c a t e o p e r a t i o n
which g r e a t l y a f f e c t s t h e product's q u a l i t y .
To d a t e , t h e r e has been l i t t l e work c a r r i e d
o u t on t h e d e s c r i p t i o n of t h e i n t e r n a l s t r u c t u r e .
and t h e mechanisms brought. i n t o play during t h e
drying s t a g e , The purpose o f . t h i s study i s t o
analyze serum t r a n s f e r s ( water+solutes) i n a.natura1
.rubber sample with t h r e e experimental methods:
I ) k i n e t i c s of drying ; 2) measurement d u r i n g t h e I
k i n e t i c s t u d y , of t h e t e m p e r a t u r e ' d i f f e r e n c e
between t h e s a m p l e and t h a t of t h e drying
atmosphere; 3 ) measurement of t h e f r e e z i n g p o i n t
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temperature of t h e serum contained i n t h e product ,
a t d i f f e r e n t s t a g e s of t h e d r y i n g k i n e t i c s ,
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2; EXPERIMENTATION
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2.1 Sample p r e p a r a t i o n
The latex used,whose dry rubber content(DRC)
i s 37% , comes f r o n the Ivory Coast , Latex i s then
d i l u t e d with water i n o r d e r t o o b t a i n a 15%
r e f e r e n c e DRC
Coagulation i s obtained by adding
a q u a n t i t y of a c e t i c a c i d with a 4 . 8 PH. The
coagulum obtained r e q u i r e s a maturation p e r i o d of
about 20 hours during which a rubber p a r t c c l e
p a r t i a l s o l i d i f i c a t i o n t a k e s place.The coagulum
i s then laminated and washed t o o b t a i n a n average
10 mm t h i c k s h e e t , f r o m which w i l l be taken a 38 mm
diameter sample with a punch. Before c o a g u l a t i o n a
thermocouple is i n s e r t e d a t . t h e c e n t e r of t h e
sample.
.
2.2 Experimental device d e s c r i p t i o n
;
.
The measurement c o n t a i n e r c o n s i s t i n g i n a
beaker i s l o c a t e d i n a thermostable b a t h ( F i g . ] )
tthermo-regulated with an immersed thermostat.
P a r t i a l p r e s s u r e w i t h i n t h e beaker i s maintained
steady with aqueous s u l f u r i c acid.The thermocouple
placed i n the sample measures t h e Tfltemperature,
a second thermocouple l o c a t e d c l o s e t o t h e sample
measures the temperature of the d r y i n g a i r T a .
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2.3 Operating procedure
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The sample i s placed i n t h e measurement
container
A s t h e experimentproceeds the sample
mass and Ta and TE temperatures a r e measured. A t
r e g u l a r time i n t e r v a l s f r e e z i n g t r i a l s a r e
performed. The sample i s taken o u t of t h e
measurement c o n t a i n e r and placed i n a r e f r i g e r a t i n g
Wc,meanwhile t h e thermocouple a t t h e
system a t
c e n t e r of t h e sample measures i t s temperature
progress.,P,rior t o and following t h e f r e e z i n g
operation
weighings were taken t o check t h a t no
s i g n i f i c a n t mass loss occured a s a r e s u l t
The
f r e e z i n g o p e r a t i o n which l a s t s 20 minutes on
average does n o t s i g n i f i c a n t l y a l t e r the sample
drying process
The experiments were performed
under t h e following condition$ : Ta = 4 9 ' C ;
r e l a t i v e humidity of t h e a i r hr=47%. A t t h e end
of t h e experiment
t h e sample reaches i t s s t e a d y
s t a t e mass
i t i s then thoroughly dries above
a concentrated s u l f u r i c a c i d s o l u t i o n f o r c i n g t h e
r e l a t i v e humidity of t h e a i r t o be lower than
p.01 % ,
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3.EXPERIMENTAL RESULT ANALYSIS
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3.1 Dryingkinetics.Variation of t h e temperature
d i f f e r e n c e between the drying a i r and the sample
F i g u r e 2 shows t h e k i n e t i c s of drying of t h e
rubber sample as w e l l a s t h e temperature
d i f f e r e n c e between the dry a i r Ta and Khe sample
Tf, over time. Two drying periods can be observed
from t h e water c o n t e n t changes; the f i r s t , c a l l e d
"constant r a t e period",between 80% and 33% ; t h e
second between 33% and 2.5% during which t h e
t h e drying speed markedly slows down i s r e f e r e d
t o a s t h e d i f f u s i o n period [2] , [3J. The Ta-Tf
d i f f e r e n c e over t i m e confirms t h a t two drying
periods occur. Once the product temperature i s
set
Ta-Tf temperature d e v i a t i o n s t a b i l i z e s a t
9.8"C f o r a c o n s t a n t r a t e drying p e r i o d then
decreases f o r a d i f f u s i o n period u n t i l reaching
a low v a l u e (Ta-Tf = 0.3OC) .During t h e c o n s t a n t
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r a t e period, t h e sample s u r f a c e i s . s a t u r e d with
serum,a)heat and mass t r a n s f e r balance occurs on
t h e product s u r f ace, To t h i s Balance corresponds. a
w e t a i r temperature Tw which f o r t h e above
mentioned a i r c h a r a c t e r i s t i c s i s 38OC; t h i s
temperature i s c l o s e t o t h e Tf v a l u e measured (TE='
38.7%).
3.2 Analysis of t h e f r e e z i n g p o i n t temperature
lowering of t h e serum contained i n t h e sample
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F i g u r e 3 shows t h e r e f r i g e r a t i n g temperature
v a r i a t i o n s over t i m e of t h e sample a t di'fferent .. .
drying moments. TWO curve t r e n d s can lie observed,
The f i r s t which corresponds t o curves CJ t o C,i's.
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c h a r a c t e r i z e d by temperature l e v e l i n g s , the .
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f r e e z i n g ' t e m p e r a t u r e i s p r a c t i c a l l y steady and
equal t o -4.2"C. The second one (curves C and C )
i s c h a r a c t e r i z e d by temperature rises l i n z e d t o
appearance of overfusion., These r i s e s p r o g r e s s i v e l y
l e v e l off due t o t h e water c o n t e n t d c r e a s e of the
fi
sample. For a water c o n t e n t Felow 7X no
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c h a r a c t e r i s t i c s i g n a l making ï t p a s s l b l e t o d e t e c t
the change from t h e l i q u i d to s o l i d s t a t e of s e r u m
can be observed (C ) , Overfusion a p p e a r s f o r a
water c o n t e n t i n c l i j e d between 53% and 338.
Overfusion seems t h u s t o determine the l i m i t
between t h e two mass t r a n s f e r mectiani'sms n o t i c e d
i n f i g u r e 2. This t r a n s i t i o n could €i&
associated :
with a p o r a l space s h r i n k i h g on t h e sample surface!.
pores t a k i n g t h e so c a l l e d "ink. b o t t l e " shape which.
favours t h e appearance of overfusion[4].
.recovered during s y n e r e s i s , showed t h a t t h e
lowering of t h e f r e e z i n g p o i n t i s , i n this c a s e ,
a p n r o x i r a t e l y 4-C. '?hiS.value being more o r less
equal t o t h e lowering of t h e f r e e z i n g p o i n t of t h e
serum contained i n t h e sample during t h e c o n s t a n t
r a t e period, t h e i n f l u e n c e of t h e p o r a l space,
during t h i s ,phase, can be considered n e g l i g i b l e :
8220. I n t h e s e c o n d i t i o n s , t h e r e l a t i o n (2)
.becomes :
m e r e n* i s t h e mole number of s o l u t e s p e r
,kilograms of 'dry rubber.
.Figure 5 ahows t h e n,* .vaiiat.iop according t o
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3 , 3 Comparison of t h e t h r e e me.thods..
Figure 4 shows a s y n t h e s i s of t h r e g a n a l y s i s
methods. According t o t h e water content; the drying
rate dw/dt, th&,.Ta-.Tf temperature d i f f e r e n c e and '
'
t h e lowering of t h e f r e e z i n g poïnt.0 '1 T.-#'-T-.
(TO=213'K
T being t h e f r e e z i n g temperature: o f
serum), a r e shown i n t h i s f i g u r e which a l s o .
i n d i a a t e s t h e t h r e e methods concur i n locati'ng a t
about 40% t h e water c o n t e n t during t h e t r a n s i t i o n
from a consdant r a t e p e r i o d dryi'ng t o di'ffusion
drying.
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3.4 Analysis of t h e a o l u t e c o n c e n t r a t i o n , v a r i a t i o n ;
of serum through cryometri'cs
,4.CONCLUSION
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The t h r e e experimental methods s e t i n
j o p e r a t i o n c l e a r y show two drying phases of rubber
j c h a r a c t e r i z e d by v e r y d i f f e r e n t t r a n s f e r
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mechanisms a r e w e l l understood during t h e , . .
j c o n s t a n t r a t e period, those, occuring during t h e
j
d i f f u s i o n phase are s t i l l t o be b e t t e r defined.
'REFERENCES
I
LE BRAS J., Elements de Sciences et : r '
'Technologie du Caoutchouc, 1951.
! [Z]
GALE R . S . , A survey f a c t o r s involved i n an
/ e x p e r i m e n t a l study of t h e drying of s h e e t rubber.
,J. Rubb. Res, I n s t . Malaya, 16, 38, 1959.
' [3]. GALE R.S., Drying of s h e e t rubber i n t h e
f a l l i n g r a t e period. Trans,. I n s t n . Chem. Engrs.,
15, 150, 1962.
[4]. H0MSHAI.I L.G., L'eau e t l e s s o l u t é s dans l e s
milieux poreux: Etude p a r m i c r o c a l o r i m é t r i e 1
basse température de l ' e f f e t des s u r f a c e s e t de
l a p o r o s i t é , Thése de Doc-Ing., U n i v e r s i t é de
P a r i s V I I , 1980.
[s] ..GUGGENHEIM E.A., Thermodynamique, Dunod,
P a r i s , 1965.
.
. (3 = Q1+!82
.
.(I.)
On t h e s u p p o s i t i o n the. serum behaves a s an
i d e a l d i l u t e d s o l u t i o n [5] :-
Where R i s t h e c o n s t a n t of perfec! gases, To
t h e f r e e z i n g temperature of pure water, Lo t h e
l a t e n t f r e e z i n g h e a t , Mo t h e molar mass of pure
water, ms the s o l u t i o n m o l a l i t y .
.
.40%, a l i n e passing through t h e o r i g i n i s
'
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j o b t a i n e d , thus v e r i f y i n g r e l a t i o n (3). During
, c o n s t a n t r a t e drying, t h e c o n c e n t r a t i o n of s o l u t e s
i n t h e serum w i t h i n rubber, remains c o n s t a n t , t h u s
explaining a mass t r a n s f e r of serum towards t h e
s u r f a c e . The experiment d e v i a t i o n when compared
t o r e l a t i o n (3) during t h e d i f f u s i o n phase (w<40X)
can be explained i n t h r e e ways: 1) p o r a l space
i n f l u e n c e ( b f O ) , 2) t r a p p i n g of t h e s o l u t e s i n
r s t r u c t u r e , 3 ) water evaporation and t r a n s f e r i n t o
!gaseous phase towards t h e s u r f a c e , which would ' be
' accompnied by an enrichment of t h e serum i n
s o l u t e s . C e r t a i n s t u d i e s [ZJ tend t o show t h a t
: r u b b e r remains s a t u r a t e d i n l i q u i d phase during
the whole drying period, which would e n t a i l t h e
- e l i m i n a t i o n of t h e t h i r d hypothesis. The c u r r e n t
r e s e a r c h aims t o a s s e s s a l l t h r e e hypotheses i n '
o r d e r t o d e s c r i b e t r a n s f e r s during the d i f f u s i o n
phase.
I [I].
The lowering of ttie f r e e z i n g p o i n t of t h e . 9
serum contained i n ruS6er can Lie due t o t h e ' ' . .
presence of s o l u t e s and t o t h e p o r a l space
s t r u c t u r e . I f @ p l i s t h e c o n t r i S u t i o n due t o tlie
presence of s o l u t e s and t h e D i t h a t due t o t h e
p o r a l space, t h e n ?
Measurements taken on t h e f r e e serum
fk. 'For water contents g r e a t e r than approximately
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ACKNOWLEDGEMENT
This r e s e a r c h was c a r r i e d o u t with support of
GRECO CNRS: Drying and t r a n s p o r t mechanisms i n
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nun-satunated porous media, and of the Coordinated
Regional Programme for the Drying of crops and Food
Products.
liisulating Ileaker
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Thermometer
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Immersed
thermostat
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sample
solution
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Figure 1 : Diagram showing the rubber sample drying experimental
device.
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60
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LO
40
bt
c7
3
5
20
O
24
48
72
96
120
144
168
192
Time, hours.
Figure 2: Variations of the sample water content and the temperTture difference
between the drying air and the sample over time. The subscript letter
Ci identifies the different moments when the sample is.taken out for freezing.
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-7.5
-10
-12.5
-15
-17.5-20
Figure 4 : C h a r a c t e r i s t i c curves obtained when
drying a n a t u r a l rubber sample.
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l
15
10
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20
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T h e . alnutel
Figure 3: Cooling temperature v a r i a t i o n over
t i m e a t d i f f e r e n t moments of t h e drying k i n e t i c s
of a n a t u r a l rubber sample. T h e s u b s c r i p t l e t t e r s
C i i d e n t i f y t h e d i f f e r e n t moments when t h e
sample is taken o u t f o r f r e e z i n g .
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20
30
40
50
60
70
80
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Figure 5: Variation of mole number of s o l u t e p e r kilograins of dry rubber
i n the sample according t o t h e water content.
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