Pertanika 5(2), 192-195 (1982)

Determination of Dry Rubber Content of Hevea Latexby Microwave Technique1

KAIDA BIN KHALIDJabatan Fizik, Fakulti Sains dan Pengajian Alam Sekitar, Universiti Pertanian Malaysia,

Serdang, Selangor, Malaysia

Key words: Dry rubber content; Hevea latex; microwave technique.

RINGKASAN

Pengukuran bagi berat getah kering lateks hevea dengan menggunakan teknik gelumbang mikroadalah diterangkan. Dalam kaedah ini pengecilan gelumbang mikro pada 1017 GHz bagi lateks diukur danditentu ukur dengan kaedah makmal piawai. Dibandingkan dengan kaedah piawai, alat ini berkeupayaanmemendekkan tempuh pengukuran dari 8 - 16 jam ke tiga minit. Pekali korelasi diantara berat getah keringyang diukur menggunakan alat ini dan kaedah makmal piawai adalah 0,998 dan sisihan lazim adalah kurangdari 0.7%.

Keterhasilan semula bagi kaedah ini adalah pada paras 0.8% unit berat getah kering.

SUMMARY

The measurement of the dry rubber content of the fresh hevea latex by microwave technique isdescribed. In this technique the attenuation of microwave by latex is measured at 10.7 GHz and is thencalibrated against the standard laboratory method. Compared with the standard method this instrument isable to reduce the measuring time 8-16 hours to three minutes. The correlation coefficient between theDRC determined using this instrument and the standard laboratory method is 0.998 and the standarddeviation is less than 0.7%.

The reproducibility is at a level of 0.8% unit DRC.

INTRODUCTION

Hevea latex is a biological product of acomplex composition. The basic components of afreshly tapped natural rubber latex, other thanwater which constitutes about 22 to 48%, are dryrubber (20 to 45%), proteinous substances (1.5%),resinous substances (2%), carbohydrates 1%,inorganic matter 0.5% and other (CHIN, 1979).Therefore transaction with the tappers depends onthe dry rubber content (DRC) of the latex and thetrue DRC of the latex must be determined toensure a fair price.

Many methods for the determination of theDRC of field latex have been developed and onlythree methods have been widely used. These arethe standard laboratory method, the 'Chee'

method and the hydrometric method. Thestandard laboratory method is based on Malaysianstandard MS 3:35 : 1975 (SIRIM, 1975) and thismethod is commonly used in research and qualitycontrol laboratories. This technique, however, isnot suitable for field use because of high capitaloutlay. The whole operation makes more than16 hours and requires skilful operators. The 'Chee*method is actually an over simplified version ofthe standard laboratory method. Although thecapital outlay is small and the total error is approxi-mately 1% compared to the standard method theoperational procedure is still slow. The hydro-metric method is widely used in the field becauseof its speed and operational simplicity. Howeverthis method has a coefficient of deviation of 11.7%in total error based on the standard laboratorymethod

The technique described in this paper was judged the best entry in the international competition on DRC determinationorganised by the Malaysian Rubber Research and Development Board held recently.Key to author's name: K. Kaida

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An attempt is made to find an alternativemethod of determining the DRC of field latexwhich is accurate, and rapid, and the instrument issimple to operate, cheap and portable.

In this article, a measuring instrument basedon a microwave technique is described.

Physical PrinciplesThe principle of microwave technique is based

on the fact that at microwave frequencies the losstangent and dielectric constant of water are muchhigher than the non-water substances in the latex.

Normally at room temperature and at10.7 GHz the value of the dielectric constant e ofwater is about 60 and the loss tangent tan 5 isabout 0.54. The solid content of the latex has adielectric constant in the range of 2.0 to 2.5 andthe loss tangent is between 0.01 to 0.05.

This means that the attenuation due to wateris large while that caused by dry rubber is small.Therefore, the higher the total solid content thelower the attenuation of the microwave power.

By assuming the proportional relationshipbetween the total solid content and DRC, acorrelation between the DRC and the attenuatedmicrowave power can be derived.

Design of The DRC Meter

This technique is based on the same principleused for measuring moisture content. Instead ofmeasuring the water content, we measure the solidcontent of the material. Figure 1 shows a blockdiagram of the microwave DRC meter. The meterconsists of a Gunn diode oscillator, transmitting

Fig. 1. Block diagrammeter.

of the Microwave DRC

and receiving horns, and a detector. A microwavesignal at 10.7 GHz, 10 mW is generated by theGunn diode which is located in a rectangularwaveguide cavity. The detector is made up of areceiver horn, coaxial adapter, detector diode andan amplifier. In order to measure the attenuationthe transmitter horn and receiver horn are setfacing each other in a horizontal plane. The latexis filled in a perspex cell and placed in a horizontalposition in the gap between these horns. The cellis placed in a horizontal position to ensure uniformdensity of the latex. The inner dimension of theperspex cell is 6.35 cm X 5.67 cm X 0.45 cm. Themicrowave power is measured by detector anddisplayed on the millivolt meter.

Figure 2 shows a photograph of the microwaveDRC meter.

Fig. 2. The Microwave DRC meter.

C BY STANDARD LABORATORY METHOD .%

Fig. 3. Calibration of the Microwave DRC meter:Output voltage of the meter versus DRCby standard laboratory method.

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DETERMINATION OF DRC OF LATEX BY MICROWAVE TECHNIQUE

Method of MeasurementTo calibrate the meter, the correlation between

the actual DRC (measured by Standard LaboratoryMethod, MS 3:35:1975) and the output voltageof the meter was obtained. Initially, the cell con-taining distilled water is placed in the gap betweenthe horns and the amplifier and adjusted to givezero output. The distilled water is then replacedwith the latex and the output voltage (representingthe attenuation due to absorption) is recorded.For the measurement there should be no gasbubbles in the cell and the outer part of the cellshould be dried. The DRC of the latex sample isthen determined using the Standard LaboratoryMethod1. A calibration curve of the microwaveoutput voltage against the DRC is then plotted asshown in Figure 3. This calibration curve can sub-sequently be used with the instrument to ascertainthe DRC of the latex. Apart from the DRC, thetotal solid content of the latex can also becalibrated.

Experimental ResultsVarious latex samples were taken from the

University farm. Samples with lower DRC wereprepared by diluting with distilled water. Theresults for the DRC ranging from 0% to 53% areshown in Figure 3. The temperature for thesemeasurements ranges between 20° to 30°C.

Taking Y to be the value measured by thestandard method and X to be the value measuredusing this instrument (in millovolt), the linearformula is log Y = log 6.3 + 0.514 log X. Thisequation is obtained by using a least squarescurve fitting. The sum of square errors is 6.3.This equation was used as a calibration curve.

PERFORMANCE OF THE INSTRUMENT

Accuracy and reproducibilityThe present technique is able to reproduce

duplicate results to a level of 0.8% unit DRC.The deviation of the test result is less than 0.7%compared to that obtain by the standard methodand is quite satisfactory for practical use.

Figure 4 shows a relation between the DRCobtained by the microwave method and theStandard Laboratory Method for 29 samples oflatex. The correlation coefficient between theresults of the methods is 0.998. There is, however,a total error of 0.2% in the Standard Laboratory

Method. This deviation may be due to thedifferences in the chemical compounds of thelatex, instability of the microwave power,detector sensitivity, moisture content of the air,room temperature and the dryness of the outerpart of the celL

7 0

6 0

5 0

40

3 0

20

10

1

_

-

0 /

/

' 0

/

1 '

0

%

•

•

or

4m

/

/

//

/

-

-

-

-

u 10 20 30 40 50 60 70

(MC • Y gT AND AW) LAB, MKTHOO (%)

Fig. 4. DRC indicated by the microwave methodversus DRC by the standard laboratorymethod.

Figure 5 shows the stability characteristics ofa microwave signal with respect to time for fivesamples. The degree of stability increases as the

11 1* 30 1* ! • 31 »

Fig. 5. Time dependence of the output voltage.

1 Standard Laboratory Method : MS 3 : 35 : 1975 Briefly the method involves acid coagulating of a known weight of fieldlatex and heated over a steam bath until a clear serum is obtained. The coagulum is thoroughly washed and placed in anoven at 70°C over night before reweighing.

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time increases and the measurement can be takenafter about 2 seconds.

SpeedThis method is rapid and the whole operation

Possibly the error can be reduced by using theamplitude modulated signal so that a more stablemicrowave power can be obtained. Instead oflooking at a variation of the attenuation with DRCwe can also use the variation of phase shift with

takes less thentiming chart of

Operation

The timerequired(seconds)

three minutes. Table 1the whole operation.

pouringthe latexin dry cell

10

Washingdrying theouter part of

cell

60

shows the DRC. Thisinvestigation.

TABLE 1

Timing Chart

Taking the readingPutting the (+stabilizingcell in the time)

gap

5 30

aspect is

Calibration

15

currently undergoing

Removingthe cell

10

Totaltime

130 sec.

Ease of the OperationThe method is easy to operate and does not

require skilful operators. It is easier than theHydrometric Method. No chemical reagents areneeded for the test. The instrument is portable andoperates on D.C.. Scale reading is easy especiallywhen using the digital version. The tapper is ableto witness the whole operation of the test on theirlatices and obtain the result after three minutes.

Capital OutlayThe cost of the instrument is about one

thousand dollars (M$ 1,000), which may be reducedto a range of M$500 to MS800 if mass produced.

Since there is no chemical reagent neededthere will be no daily expenditure except for thechanging of the batteries.

CONCLUSIONS

The microwave DRC meter overcomes thelimitations of the Hydrometric method and 'Cheemethod and enables efficient measurement, withease of reading when the result is displayeddigitally. This technique is able to reduce themeasuring time from 16 hours to three minuteswith a deviation less than 0.7% unit DRC.

, ACKNOWLEDGEMENT

The author wishes to thank Dr. Mohd. Yusofbin Sulaiman, Head of Physics Department, U.P.M.for useful comments on the script and also JamilSuradi and Samsudin Mahmood for the initialdiscussion of the project. He would also like tothank the technical staff namely Abdullah b.Mohd., Abd. Rahim b. Abd. Wahab and Roslim b.Mohd. for their help in determining DRC by theStandard Laboratory Method.

REFERENCES

CHIN HONG CHEAW (1979): Method of Measuring theDry Rubber Content of Field Latex. RRIM TrainingManual on analytical Chemistry. Latex and RubberAnalysis. RRIM. Kuala Lumpur. 63.

STANDARD and INDUSTRIAL RESEARCH INSTITUTEOF MALAYSIA (1975): Methods of sampling andtesting. Concentrated natural rubber latices. MS 3:35.

KRASZEWSKI, A. (1973): Microwave Instrumentationfor Moisture Content Measurement. / MicrowavePower. 8(3/4). 323 - 335.

KRASZEWSKI (1980): Microwave Aquametry - Areview. Microwave Power. 15(4). 209 - 220.

(Received 26 March 1982)

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