Pertanika 11(2), 289-297 (1988)

Microwave Drying of Hevea Rubber Latex andTotal Solid Content (TSC) Determination

KAIDA BIN KHALID, ZAIDAN B. A WAHAB and ABD. RAHMAN KASMANIDepartment of Physics

Faculty of Science and Environmental Studies,Universiti Pertanian Malaysia,

43400 UPMf Serdang, Selangor, Malaysia.

Keywords: Microwave drying; Hevea rubber latex; Total Solid Content (TSC)

ABSTRAKKertas ini membicarakan pengeringan mikrogelomhangm bagi lateks getah hevea. la menggariskan

secara teori dan eksperimen kesan jumlah berat bahan (5 gm hingga 15 gm), paras kuasa mikrogelombang(3W/cm* hingga 10.6W/cm3) dan kandungan pepejal awal (25% hingga 60%) terhadap kadar pengering-an serta jumlah masa pengeringan. Suatu siri graf (jisim lawan masa pengeringan) dilakarkan dan keadaanoptimum pengeringan ditentukan. Perhubungan yang baik di antara teori dan eksperimen telah diperolehi.Kadar pengeringan dan jumlah masa pengeringan untuk jisim bahan 5 gm hingga 15 gm dengan TSC=39.2% pada kuasa 9.3 W/cm* masing-masing lebih kurang 0.07gm/saat dan 4 minit. Jisim bahan >20gm,dielak bagi menghalang berlakunya letupan dan rencikan semasa proses pengeringan. Dicadangkan jisimdan kuasa optima pengeringan masing-masing adalah 10 gm dan 9.3 W/cm3. Keputusan eksperimen me-nunjukkan dengan jelas bahawa pengeringan mikrogelombang amat efisen bagi pengeringan lateks getahhevea jika dibandingkan dengan kaedah biasa yang memerlukan 1 hingga 2 jam.

ABSTRACTThis paper deals with the microwave drying of the hevea rubber latex. It outlines the theoretical

and experimental aspects on the effect of sample weight (5 gm to 15 gm)f microwave power levels (3W/cm3 to 10.6W/cm*) and initial TSC (25% to 60%) to the drying rate and final drying time. A seriesof drying curves (mass versus drying times) was generated and optimum conditions for drying weredetermined. A close relationship between theory and experiment has been found. The drying rateand final drying time for 5 to 15 gm samples with TSC = 39.2% at 9.3 W/cm* is approximately 0.07gm/sec and 4 minutes respectively. A large amount of sample > 20 gm is avoided to prevent any ex-plosion and sputtering of the sample during drying process. It is suggested that the optimal mass andpower level are 10 gm and 9.3 W/cm3 respectively. The experimental results clearly showed that micro-were drying is very efficient for drying of fresh hevea latex as the conventional drying method needs1 to 2 hours.

INTRODUCTION convenience and controllability of the process, •;" '' u and high heating efficency. The latter is due to

Microwave heatine and drying processes have r~ ~ *, 5 , . A -1 r direct heating whereby surrounding air and equip-

been well developed in various industrial apph- • i, , ^ . r .. • *ua ment remain cool,

cations and the most popular application is in trieuse of microwave ovens for domestic and commer- i n th e rubber industry, microwave heatingcial heating. and drying found extensive application in pro-

The main advantages of micro drying are cessing of rubber products (Shute, 1971) Micro-short time required for drying, uniform drying, wave power is used in several processes such

Key to authors' names: K.B. Khalid, Z.A. Wahab & A.R. Kasmani

KAIDA BIN KHALID, ZAIDAN B. A WAHAB AND ABD. RAHMAN KASMANI

as rubber moulding, extrusiohs curing and beltingpreheater. Vosilakos et al (1984) investigatedexperimentally the microwave drying charac-heristics of various polymers such as polyisoprenerubber, polyolefin, polypropylene, polyethyleneetc, with all initial moisture content about 40%.They showed that there was a good potentialin the application of microwave drying.

However, until now, only limited workhas been carried out on microwave dyring of thefresh hevea rubber for determination of TSC.This quantity is a prime factor in determiningthe quality of the hevea rubber latex.

The basic components of fresh hevea otherthan water (about 50%-80%) are 18% to 45%dry rubber content (DRC) and approximately2% to 5% non-rubber constituens or total solidcontent of about 20% to 50% (Chin, 1979)(Fig. 7). Non rubber solids such as proteinoussubstances, resionous substances, carbohydrates,inorganic matter and others constitute about 4%(Cook, et aly 1953). Therefore the knowledge ofthe TSC can be used to determine dry rubbercontent of the latex.

Moisture Contentor Water (50-80%)

[Non-Rubber Solid^_(2-5%)

4Rubber Hydrocarbon^(DRC) (18-45%)

Fig. 1: Basic components of hevea rubber latex

The conventional oven has been widely usedfor TSC or DRC determination, but for a highaccuracy level it takes 1 to 2 hours. The use of alow microwave power level for determination ofTSC or moisture content of hevea rubber latexhas successfully been done. (Khalid, 1982, Khalid

1983, Khalid 1987). Despite the complexity ofthe physical nature of the hevea latex, thereexists a relationship between moisture contentand attenuation of the microwave power. Theaccuracy of the measurement is less than 1% perunit moisture content.

In this paper the impact of microwavedrying on increasing the efficiency and dryingtime of fresh hevea latex are studied. It includestheoretical and experimental investigations on theeffects of initial mass, microwave power level andinitial moisture content to the drying time.Optimization in the efficiency of the dryingprocess will be discussed.

Physical Processes

The physical process which govern the removalof moisture from a liquid can be divided intotwo stages i.e. initial heating up period and dryingprocess as shown in Figure 2,

In the initial heating up period process,the temparature of the material rises towardsthe boiling point of the liquid. The time takenfor this process is given by

At = (£- hATo (1)

where h is the specific heat of the materialP/V average volumetric power absorption and,AT is the temperature rise and p is the density ofthe material.

initial - •mass

final mass-*

i

1

t

ii11IIi

I1I

XV1

I -II -

III -

- m

- heating-up period- constant drying

rate period- Falling drying

rate period

Drying time

Fig. 2: Drying characteristics of a liquid

290 PERTANIKA VOL. 11 NO. 2, 1988

MICROWAVE DRYING OF HEVEA RUBBER LATEX AND TSC DETERMINATION

In the drying process, the rate of moistureevaporation of the liquid takes places under twodistinct periods, ie. constant drying period andfalling drying period.

During the constant drying rate period, themoisture content is very high and the evaporationwill occur from the surface at a contant rate.Under steady state conditions, the heat transferedfrom microwave power is exactly balanced by thelatent heat of water.

(2)

The moisture content (wet basis) at time tis determined by using the following expression:

Mt f (5)

where X ^ is the mass of water at time t.

X$t is the mass of sample at time t.

The dielectric properties of hevea rubberlatex at a particular time t is determined by usingKraszewski model (Kraszewski, 1976).

where X is the latent heat^m is the mass of water evaporated duringperiod &t.Vs is the volume of the sample.During the falling rate period the drying

process follows the conventional convectivedrying.

The average power absorbed per volume bya material exposed to electromagnetic fields as aresult of dielectric losses is given by (Stuchlyetal 1972).

t - oE2 - e ' e E2 t a n 6 2irf (3)V i o 1

where a represents the conductivity, e1 is therelative permittivity, e

o is t n e permittivity ofvacuum, E- is the rms electric field strengthinside the material, tan5 is the loss tangent andf is the frequency of the microwave radiation.

The magnitude of the Ej can be relatedto the magnitude of the electric field outsidethe volume of the sample, EQ. Von Hippel (1954)suggests that the standard result of a particularshape of a dielectric field is given by

w / e (4)

where w is the shape correction which isdetermined by the geometry of the polarizedbody. It can be calculated for spheres, cylindersand ellipsoids. The value of w for the case of thelatex in the cylindrical beaker is given in •the Appendix.

The drying time for each sample was calcu-lated and corrected with the duty cycle of themicrowave power. A simplified flow digram forthe cumulation of successive time incrementsis shown in Figure 3.

where e,

V t / £ H +

*i-

(1 - Vfc)

is the relative

! is thedielectric constant of the latex and e

dielectric loss of the latex, eM = e ' - j e " f c '

H n n nis the relative dielectric constant of the water ande 7, is the dielectric loss of the water and e n =

eR ~ is the relative dielectric constant

of the solid hevea rubber and e " is the dielectricloss of the solid hevea rubber.

Vt is the volume fraction at time t and isrelated to the moisture content Mt.

Vt = Mt/(Mt+ T[w (i

7dwhere 7^ and 7W are relative density of thesolid and water respectively and are considered tobe constant with 7^ • 0.94 and 7W = 1.0.

From our measurement at 100 C, is2.6 - j 0.02 and eH (at 100 C) equals to50-jlO (Metaxas et al, 1983). The TSC of hevearubber is computed from

TSC = (Wj/Wg) x 100 (8)

where Wg is the initial weight of the latex andWf is the final weight of the latex after drying.

Microwave heating and drying involves avery complex physical process. Various assump-tions have been made on the above calculationswithout taking account of the following para-meters or phenomena.

(i) reflection of the microwave radiation at thesurface of the sample.

PERTANIKA VOL. 11 NO. 2, 1988 291

KAIDA BIN KHALID, ZAIDAN B. A WAHAB AND ABD. RAHMAN KASMANI

readinput data

Calculate EQ andtime for initial

heating up

Make correction formicrowave duty cycle

Set timeincremental, ^ t

Calculate Amlosses due to evaporation

Calculate new mass,new moisture content

and new £ i and cl

Calculate newP/V

make correctionfor microwave

duty cycle

print totaltime, mass

and moisture content

No.

Note: Eo is determined by water-load method[Mae Latchy et. al 1980]

292

Fig. 3: A simplified flow diagram for calculation of the heating time

PERTANIKA VOL. 11 NO. 2, 1988

MICROWAVE DRYING OF HEVEA RUBBER LATEX AND TSC DETERMINATION

(ii) heat loss due to radiation and conductionduring the constant drying rate period.

(iii) movement of moisture within the materialfrom the surface during the falling dryingperiod.

MATERIALS AND METHODSA modified National (Model: NE-6760) com-mercial microwave oven operating at 2450 MHzwith a maximum measured power output of12 Watt/cm3 was used in this experiment (Fig, 4).The oven features five different power settings(100%, 89%, 77%, 52% and 23% of maximumpower). When one of these powers is set at anypower setting other than high power, the variablepower switch is energized intermittenly by signalsfrom the variable power control circuit. Thevariable power control circuit controls the ON-OFF time of the variable power switch contactswithin a specific duty cycle. One complete dutycycle of this oven is approximately, 22 seconds.ON-OFF cycle of magnetron can be detectedsimply by observing the dimming of the ovenlight. If the oven light dims the circuit is supplyinghigh voltage to the magnetron and when the ovenlight goes bright, it indicates no voltage supplyto the magnetron. The brightness of the oven lightduring the process can be detected by a photodetector. Fig. 5 shows the relationship betweenthe output power of a microwave oven and thecorresponding duty ratio.

waveguide

microwave radiation for 1 minute. The tem-perature was measured using a Cu-Constantanthermocouple immediatly after exposure.

MAGNETRONMicrowaveSourceFreq: 2.45 GHz

Charrecorder

Fig. 4: A microwave oven cavity

The oven also incorporates a metalic fan todistribute the power evenly inside the oven cavity.Fig. 6 shows the average temperature distributionwithin a 900 cm2 of the central region of thecavity. This was determined using a beaker con-taining 100 ml of water and exposed to the

— 33.8 33.6 32.4

32.8 33.8 32.6

32.6 33.8 33.8-

Fig, 6: Temperature distribution in centralregion of cavity, 3 cmx 3 cm.

The oven has a balance placed at the bottomas seen in Fig. 4. The variation of mass with timeof heating is automatically recorded on the chartrecorder. An electric balance Gibertini (ModelE415) with readability 0.0001 gm and precisionof — 0.0005 gm was used to weigh the samplebefore and after drying.

In a typical experiment, a sample of hevealatex under investigation was spread on a 500 mlcylindrical beaker, weighed and then placed intothe oven.

Before the initial amount of the sample waschosen, various samples with initial masses of40 gm, 30 gm, 25 gm and 15 gm were tested. Theresults of this testing is shown in Fig, 7 withaverage microwave power of 10.6 W/cm3. Forinitial masses greater than 25 gm, the samplesexploded and the latex sputtered. This effectis more serious for an initial mass greater than40 gm and it happened soon after the heatingup period, and was repeated several times withthe duty cycle of the microwave radiation. Thesame effect has been seen for different levels ofmicrowave power. From this testing, a sampleweight smaller than 20 gm was chosen.

A series of solutions from hevea rubberlatex were prepared ranging in dilution from40% to 100% moisture content (wet basis). Thehevea latex used in this experiment came fromconcentrated hevea rubber latex (supplied by

PERTANIKA VOL. II NO. 2, 1988 293

KAIDA BIN KHALID, ZAIDAN B. A WAHAB AND ABD. RAHMAN KASMANI

INDICATIONOUTPUT POWERAGAINST HIGH

POWER

OUTPUT VOLTAGE OF PHOTODETECTORAND — CORRESPONDING ON-OFFTIME ON VARIABLE POWERSWITCH. (S)

AVERAGEOUTPUTPOWER

(W/cm )+ 0.2

HIGH 22/22(100%)

CN ON ON ON

12.0

MEDIUMHIGH

bright

19.5/22(89%) iJfvj^U

U ON Ll ON

10.6

MEDIUM 17/22 (77%)

17.0

ON OFF ON OFF ON OFF ON

9 . 3

MEDIUM

LOW11.5/22(52%)

11.5

OFF ON OFF ON OFF ON OFF ON

6 . 3

LOW 5/22 (23%)

OFF DN OFF OM OFF K)N O F F

3.3

Fig. 5: Relationship between the output power of microwave oven and the corresponding duty ratio

294 PERTANIKA VOL. 11 NO. 2, 1988

MICROWAVE DRYING OF HEVEA RUBBER LATEX AND TSC DETERMINATION

•1

1

iijj

ba

lan

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mA

o 1

>lt

ag

e

a

out

)

fJ•

i output

bright;! y

1 i • • ! •

i ' 22 seco- / " " « r.

volta'pe bf photodetector whilei*etocting ovon l ight

1 > A

. : . 1: • • t

¥ !' • . ! • ' • •

!§§1• I : i • j ' 1 • j . i i . . . t ( : : ' | •, : •

• '; • 1 corresponding ON-OFF. .' time Of microwave poweri i i j i i ; i

•jrfi *

jliiiiij"III:fill I! i'iti! !

' I : ' " :' i i; • ' i l l 1

ililjljrli • •

• : ! : , : ! i !

| 1 ; ; i : ' •

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!̂ ' 1 ;ttiiii

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| I :

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20 gM :

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i ' i i1 ,-

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|i iiiTTTttm; ; | : . i • i l l iTYsiJ ii iIT f:7

I'M

r r r ••

pit|l1

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pifj!

j I ; . ' ! ' '

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; 1 • . . • ; .

1 ! ' '

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11; " ! j j

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i

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1 i1

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• i ' • i

• ! ' ! i

1 i *

! jT:

#! 11 ";

1n imi

i f t r: »'. !

' fl

1

• %v«

I i I *«

i! Hii \ !

1;.i :

I I s i! • I 1 1 : !1 • l ;

. • . i 1 .

I

! ; •

1

H !̂ :

| ] |3

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il1;

k( • 12W/ca .

"l0.6 v

- - .

i j i i ; i : '

11. Hi..ij?

1

•

• f j

' • ' ; 1

! • •

• ; 1

: ! I ;

ji.

r/cm

I..;.i 1

i •1

r T"! ! ; •

•fiiij:ill

jjl'i

H

Fig. 7: Drying characteristics ofhevea rubber latex at various sample amounts with average

power level 10.6 W/cm3 and initial TSC =39.2%.

Rubber Research Institute Malaysia) and fresh latex are summarized in Figs. 8, 9 and 10. Thehevea rubber latex (from the university farm). drying curves were generated by plotting mass

versus drying time. All figures show a good agree-ment between theory and experiment.RESULTS AND DISCUSSOIN

The experimental data and theoritical prediction Fig. 8 show the variations for various initialof the microwave drying of the hevea rubber masses ranging from 5 gm to 15 gm average

PERTANIKA VOL. II NO. 2, 1988 295

KAIDA BIN KHALID, ZAIDAN B. A WAHAB AND ABD. RAHMAN KASMANI

16

14

12

10

Average microwave power = 9 3 W / c m

In i t ia l Total Solid Content = 39 • 2 %

theoryexperiment point

60 120 300 360

Fig. 8:

180 240time/sec

Drying curves ofhevea rubber latex at varioussample amounts with average power level9.3 W/cm3 and initial TSC = 39.2%

microwave power about 9.3 W/cm3 and TSC =39.2%. All the drying curves follow the dryingcurves shown in Fig, 2. For a particular setting ofmicrowave power, the overall drying rate increaseswith increasing amount of hevea latex. At theconstant drying period, the rate of drying forall samples is almost the same at the level of0.08 gm/sec. The final drying time is almostproportional to the initial mass of the sample.

The variation of mass with drying time forvarious power setting is shown in Fig. 9. For a10 gm sample with TSC of 39.2%, the overalldrying rate increases with increasing power level.The effect is more pronounced at lower powerlevel with the slower rate of drying. There appearsto be an optimum level, above which furtherincreases in power input results only in a smallincreases in the rate of drying and final dryingtime. For example at 6.3 W/cm3 the drying rateis 0.07 gm/sec. and is comparable to that at 9.3W/cm3 but on the other hand, it was far superiorto that at 3.3 W/cm3 with drying rate of only0.03 gm/sec. It takes from only 2 to 4 minutesto dry 5 to 15 gm samples at 9.3 W/cm3 and 10.6W/cm3, whereas approximately 8 minutes arerequired at 3.3 W/cm3.296

Fig. 10 shows the variation of mass withdrying time of 10 gm of sample at power level

10

initial TSC »

initial mass =

Average miciowave power

• 3.8 W/cm3

x 6.3 W/cm3

• 9.3 W/cm3

A 10.6 W/cm3

theoritical curve

30 60 90 180 210 240120 150time/sec

Fig. 9: Drying curves oflOgm samples at various powerlevels with initial TSC =39.2%.

10

8

6

i.

3

w\

Initial

•OX

•

A

^ rTSC V

64%52%

44%36%

27%theoriticat curve

*

30 60 90 120 150time/sec

180 210 240

Fig. 10: Drying curves oflO-gm samples ofhevea rubberlatex at various TSC and with average power leve9.3 W/cm3.

PERTANIKA VOL. 11 NO. 2, 1988

MICROWAVE DRYING OF HEVEA RUBBER LATEX AND TSC DETERMINATION

9.3 W/cm3 for various initial TSC. At the begin-ning, all curves started with the same drying rateand as time increased, the curves spread according-ly, to their TSC. The final drying time is almostinversely proportional to the initial TSC of thelatex.

f 1 ~]

cylindricalbeaker

oblat*•pherold ^^^- —

c m \

Fig. 11: Approximate shape of the hevea latex in thecylindrical beaker.

CONCLUSION

The performance of microwave drying for thedetermination of TSC of hevea rubber latex hasbeen demonstrated and general conclusions canbe drawn.

An optimal set of initial mass and power levelwhich maximizes the drying rate and minimizesthe final drying time can be determined theore-tically and experimentally. A large amount ofsamples are avoided, to prevent any explosion andsputtering of the sample during the process.It is suggested that optimal mass and powerlevel are 10 gm and 9.3 W/cm3 respectively.

Finally the experimental results clearlyshowed that microwave drying has overcomethe limitation of the conventional method byreducing measuring time from 1 hour to 2minutes.

ACKNOWLEDGEMENTWe are greatly indebted to the Physics DepartmentUPM for -the facilities and all members of thedepartment especially En. Roslim Mohd andEn. Nordin Abd. Kadir for their invaluable help.The authors would like to thank the RubberResearch Institute Malaysia and UPM farm forsupplying hevea rubber latex.

REFERENCESCHIN, H.C. (1979): Method of Measuring the Dry Rubber

Content of Field Latex-RRIM Training Manualon Analytical Chemistry Latex and Rubber AnalysisRubber Research Institute Malaysia, K. Lumpur.63.

COOK, A.S. and B.C. SEKHAR (1953): Fraction brownHevea Brasiliensis latex centrifuged at 59,000 g./. of Rubber Institute Malaya, 14: 63

KHALID, K.B. (1982): Determination of dry rubbercontent of hevea latex by microwave technique,Pertanika, 5(2): 192-195.

KHALID, K.B. and ABD. WAHAB M.B. (1983): Micro-wave attenuation of fresh hevea latex, / . RubberResearch Inst cf Malaysia, 31-3: 145-150.

KHALID, K.B. (1987): The application of microstripsensors for determination of moisture content inhevea rubber latex ( t o be published).

KRASZEWSKI, A, S. KULLINSKI, M. MATUSZEWSKI(1976): Dielectric properties and a model of diphasewater suspension at 9.4 GHz. / . Applied Physics.47(4): 1275-1277.

METAXAS A.C. and R.J. MEREDITH (1973): IndustrialMicrowave Heating, Peter Peregrinus and IEE,London. 60.

MACLATCHY, C.C. and R.M. CLEMENTS (1980):A simple technique for measuring high microwaveelectric field strengths, / of Microwave Power,15(1): 7-14.

SHUTE, R.A. (1971): Industrial microwave systemsfor the rubber industry, / . of Microwave Power6(3): 193-205.

STUCHLY, S.S. and M.A.K. HAMID (1972): Phycisalparameters in microwave heating process, / ofMicrowave Power 7(2): 177-137.

VAN HIPPEL, A.R. (1954): Dielectric and Waves, JohnWiley, New York, 254-255.

VASILAKOS, N.P. and F. Magalheas,: Microwave Dryingof Polymers, / . of Microwave Power 19(2): 135-144.

(Received 16 November, 1987)

APPENDIXThe geometrical shape of the latex in the cylin-drical beaker can be considered as having theshape of an oblate spheroid as seen in Fig. 11.

The thickness of the sample is approximatelyequal to 2c and the radius of the cylinder is equalto a. a and c are the major and minor axis ofthe spheroid respectively.

By assuming and electric field E parallelto c, the shape correction w is (Von Hippel1954)

iw = —"

arcsin e

where

PERTANIKA VOL. 11 NO. 2, 1988 297