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A rapid and accurate method for determining the volatile matter content of raw natural rubber
Polymer Testing 7 (1987) 317-323
A Rapid and Accurate Method for Determining the Volatile Matter Content of Raw Natural Rubber P. A . D . T. Vimalasiri, L. M. K. T i l l e k e r a t n e , S. W e e r a m a n and A . S. D e k u m p i t i y a Rubber Research Institute 'Dartonfield', Agalawatta, Sri Lanka (Received 8 August 1987; accepted 18 August 1987)
SUMMARY In order to eliminate mould contamination and other processing difficulties the percentage volatile matter (moisture) content in technically specified natural rubber (TSR) is maintained below 1.0%. Determination of the volatile matter content is carried out by the British Standard test method, which involves an over 5 h procedure and hence it is costly and inconvenient for laboratories handling several samples a day and also for factories where this test is carried out for production control purposes. This paper reports a modified BS method involving the use of a microwave oven instead of the conventional oven thereby cutting down the time taken for the test to just over 2 h without losing the accuracy of the test.
INTRODUCTION The manufacture of raw forms of natural rubber such as smoked sheets (RSS), crepe rubber and technically specified rubber (TSR) is particularly dependent on the efficiency of the drying process. The drying process involves the use of either a stream of hot air or a smoke house to remove around 20% by weight of moisture from the milled rubber coagulum until all signs of wet spots disappear completely from the surface. It has however been previously demonstrated ~ that even after complete drying, natural rubber absorbs moisture from the surrounding damp atmosphere due to its hygroscopic nature. The absorption of water or moisture by natural rubber, which is a 317 Polymer Testing 0142-9418/87/$03-50 (~) 1987 Elsevier Applied Science Publishers Ltd, England. Printed in Northern Ireland
318
P. A. D. T. Vimalasiri et al.
famous water-proofing material, has been attributed to an osmotic process. The mechanism of drying wet rubber as well as its hygroscopic nature depends mainly on the presence of polar non-rubber substances such as proteins and lipids which are natural contaminants. It has also been reported by previous workers that these fine protein fibres act as 'wicks' for the absorption of moisture through rubber, z The two major disadvantages of the presence of excess moisture in raw rubber a r e : 3,4 (a)
(b)
Excess moisture causing slippage of rubber in the mixing mill thereby reducing the ease of mastication and increasing the cost of milling and mixing. At moisture contents greater than 1% by weight of raw rubber, mould contamination leads to the darkening of light-coloured grades and consequently reduces the quality of bright-coloured products to an unacceptable level.
These disadvantages coupled with the need for high and consistent quality specifications has led to an internationally accepted standard moisture level for all TSR rubbers made for export. This standard is technically referred to as the volatile matter level ( % V M ) a n d is fixed at below 1.0% as delivered to the consumer. *-6 The hygroscopic nature of natural rubber necessitates an allowance of 0.5% moisture pick-up during transit, thereby imposing a maximum volatile matter level of 0.5% at the end of the drying process carried out by the rubber producers. Latex grades of TSR that exceed 0.5% moisture content are therefore unsuitable for export. The accurate determination of the volatile matter content by the BS test method No. 5923, Part 3, 1981, constitutes a central quality control and technical specification procedure in certifying the suitability of dried TSR for export purposes. The detailed test procedure for determining %VM consists of the following steps: (a)
(b) (c) (d) (e)
Homogenise a sample of TSR by passing it through a cold two roll mill ten times according to BS homogenisation method No. 6315, 1982. Weigh accurately a piece of homogenised rubber (approximately 10 g) milled down to a thickness of less than 2 mm. Place the sample in a conventional air-circulating oven at 100 + 5°C for 4h. Cool the sample in a dessicator at room temperature for 30 min. Re-weigh the sample accurately on an analytical balance.
From the loss in weight of the sample %VM is calculated. This accurate method of determining %VM therefore involves a 5 - 6 h
Determining the volatile matter content o f raw natural rubber
319
procedure with the laboratory responsible for testing a few hundred samples a day requiring a high equipment capability as well as many testing personnel. The test is also carried out for production control purposes in dry rubber manufacturing factories and they find the procedure lengthy, costly and inconvenient. This project attempts to retain the accuracy of the BS test method for %VM determination whilst employing a microwave oven in place of the conventional air circulating oven to reduce the duration of the oven drying time.
EXPERIMENTAL Several samples of TSR made out of both latex and scrap rubber in Sri Lanka, and containing low, medium and high levels of moisture, were selected for this test. From these samples two identical sets were taken for the %VM determination by both the conventional oven method and the microwave oven method. The test temperature was maintained at 100 + 5 °C in the conventional oven while the microwave oven was set to medium power level (50%/320 W). The details of the ovens used for this experiment are as follows. (1) (2)
Conventional oven--Gallenkamp OVB-305. Ventilated and of the air-circulating type. Microwave oven--Matsui Model No. 165 operating on five power levels. Operating frequency 2450 MHz and output 90650 W.
PROCEDURE Homogenisation, milling and preparation of all samples selected for this test were carried out according to the BS test method No. 5923, Part 3, 1981. Each of the two identical sets of samples were then placed in the two types of oven without touching each other and with identification labels. Samples in the conventional oven were heated for a fixed period of 4 h, but those in the microwave oven were heated for varying periods of time, from 10 to 100 min. For each period of drying in the microwave oven a fresh set of samples was used while the corresponding set of identical samples was dried in the conventional oven for 4 h. Samples removed from each oven were cooled to room temperature in a dessicator and then weighed accurately to four decimal places. The
P. A. D. T. Vimalasiri et al.
320
percentage volatile matter content in each sample was calculated as follows.
%VM content = W1 - W2 x 100
wl
where W~ = Initial Wt of the sample and W2 = Wt after heating.
RESULTS AND DISCUSSION The results in Table 1 and a plot of the difference in %VM (A%VM) between the two methods against time (Fig. 1) show that during the early stages of heating in the microwave oven, evaporation of the volatile components from the rubber sample is incomplete thereby leading to a significant deviation in the %VM measured by the two methods. However, an increase in the heating time leads to a narrowing of the deviation in the results obtained by the two methods and at 90 min of heating the two values are identical. Thus it appears that 90 min of heating in a microwave oven set to a medium power level of 320 W is equivalent to a 4 h heating process in an air-circulating conventional
TABLE 1 Average %VM Values of Rubber Samples Heated in the Microwave Oven for Different Periods of Time and the Corresponding %VM Values Calculated for an Identical Set of Samples by BS Method Time of heating in the microwave oven (min )
Average % VM values recorded
10 15 20 25 35 40 45 75 80 85 90 100
0.48 0-51 0.61 0.56 0-51 0.57 0-38 0-42 0-53 0-41 0-51 0-62
Average % VM by BS method A % VM 0-61 0.69 0.73 0-66 0.64 0.62 0-43 0.47 0.54 0.43 0-51 0-62
0.13 0.18 0-12 0-10 0.13 0.05 0-05 0-05 0.01 0.02 0.00 0-00
Determining the volatile matter content o f raw natural rubber
321
>
© 4J
°u 0 . 1 5
.,-t Ill 0 ©
144
"~ 0. i0
0.05
D
20
40
60
80 Drying
Time
i00
(Min)
Fig. 1. Difference in the %VM contents determined by the conventional oven method and the microwave oven method vs time of drying in the ovens.
oven at 100 + 5 °C for the purpose of moisture evaporation from natural r u b b e r to bring the % V M level to below 0-5% (Fig. 2).
CONCLUSION The d e t e r m i n a t i o n of the volatile m a t t e r content of dry rubber for technical specification of T S R could be accurately and rapidly carried out by a modified British Standard m e t h o d involving the heating of the
322
P. A. D. T. Vimalasiri et al.
O. 70
Q~ v 4J
I 4J 0 00.6
/
/
o 4J 4J
>0.50
......... iI
O. 40
O. 30
20
40
60
80 Drying
IOO
Time(Min.)
Fig. 2. Variation of the %VM contents vs time for rubber samples heated in the two ovens. (A A), %VM using a microwave oven; ( o - - - o ) , %VM using a conventional oven.
Determining the volatile matter content of raw natural rubber
323
test sample for 90 min at the m e d i u m power level in a microwave oven instead of using a conventional oven at 100 °C for 4 h.
ACKNOWLEDGEMENT The authors wish to thank Dr B. Boyd Coorey, the Research and D e v e l o p m e n t Manager, Baxenden Chemical Research Co. Ltd, Lancashire, UK, for helping in many ways while writing this paper.
REFERENCES 1. Tillekeratne, L. M. K. and Vimalasiri, P. A. D. T. (1985). J. Chem. Tech. Biotech., 35B, 117-20. 2. Tyler, S. (1987) J. Malaysian Rubber Res. and Dev. Board, 41](1), 17-20. 3. Heinish, K. F. and Nadarajah, M. (1960). Proc. Nat. Rub. Conf., KL Malaysia, Rubber Research Institute of Malaysia, 893 pp. 4. Tillekeratne, L. M. K., Perera, M. C. S. and Rodrigo, H. V. Effect of fresh water and sea water on different grades of crepe rubber. J. Plast. and Rub. Proc. and Applications (in press). 5. Anon. (1969). International Standards for Quality and Packing of Natural Rubber (The Green Book), Rubber Manufacturing Association, New York. 6. Bristow, G. M. and Sears, A. G. (1985). The effects of water immersion on the quality of raw natural rubber, NR Technol. Q., 16(2), 29-32. 7. Sarathkumara, S. J., Janz, E. R. and Tillekeratne, L. M. K. (1987). J. Chem. Tech. and Biotech., 39, 11-18.
A Rapid and Accurate Method for Determining the Volatile Matter Content of Raw Natural Rubber P. A . D . T. Vimalasiri, L. M. K. T i l l e k e r a t n e , S. W e e r a m a n and A . S. D e k u m p i t i y a Rubber Research Institute 'Dartonfield', Agalawatta, Sri Lanka (Received 8 August 1987; accepted 18 August 1987)
SUMMARY In order to eliminate mould contamination and other processing difficulties the percentage volatile matter (moisture) content in technically specified natural rubber (TSR) is maintained below 1.0%. Determination of the volatile matter content is carried out by the British Standard test method, which involves an over 5 h procedure and hence it is costly and inconvenient for laboratories handling several samples a day and also for factories where this test is carried out for production control purposes. This paper reports a modified BS method involving the use of a microwave oven instead of the conventional oven thereby cutting down the time taken for the test to just over 2 h without losing the accuracy of the test.
INTRODUCTION The manufacture of raw forms of natural rubber such as smoked sheets (RSS), crepe rubber and technically specified rubber (TSR) is particularly dependent on the efficiency of the drying process. The drying process involves the use of either a stream of hot air or a smoke house to remove around 20% by weight of moisture from the milled rubber coagulum until all signs of wet spots disappear completely from the surface. It has however been previously demonstrated ~ that even after complete drying, natural rubber absorbs moisture from the surrounding damp atmosphere due to its hygroscopic nature. The absorption of water or moisture by natural rubber, which is a 317 Polymer Testing 0142-9418/87/$03-50 (~) 1987 Elsevier Applied Science Publishers Ltd, England. Printed in Northern Ireland
318
P. A. D. T. Vimalasiri et al.
famous water-proofing material, has been attributed to an osmotic process. The mechanism of drying wet rubber as well as its hygroscopic nature depends mainly on the presence of polar non-rubber substances such as proteins and lipids which are natural contaminants. It has also been reported by previous workers that these fine protein fibres act as 'wicks' for the absorption of moisture through rubber, z The two major disadvantages of the presence of excess moisture in raw rubber a r e : 3,4 (a)
(b)
Excess moisture causing slippage of rubber in the mixing mill thereby reducing the ease of mastication and increasing the cost of milling and mixing. At moisture contents greater than 1% by weight of raw rubber, mould contamination leads to the darkening of light-coloured grades and consequently reduces the quality of bright-coloured products to an unacceptable level.
These disadvantages coupled with the need for high and consistent quality specifications has led to an internationally accepted standard moisture level for all TSR rubbers made for export. This standard is technically referred to as the volatile matter level ( % V M ) a n d is fixed at below 1.0% as delivered to the consumer. *-6 The hygroscopic nature of natural rubber necessitates an allowance of 0.5% moisture pick-up during transit, thereby imposing a maximum volatile matter level of 0.5% at the end of the drying process carried out by the rubber producers. Latex grades of TSR that exceed 0.5% moisture content are therefore unsuitable for export. The accurate determination of the volatile matter content by the BS test method No. 5923, Part 3, 1981, constitutes a central quality control and technical specification procedure in certifying the suitability of dried TSR for export purposes. The detailed test procedure for determining %VM consists of the following steps: (a)
(b) (c) (d) (e)
Homogenise a sample of TSR by passing it through a cold two roll mill ten times according to BS homogenisation method No. 6315, 1982. Weigh accurately a piece of homogenised rubber (approximately 10 g) milled down to a thickness of less than 2 mm. Place the sample in a conventional air-circulating oven at 100 + 5°C for 4h. Cool the sample in a dessicator at room temperature for 30 min. Re-weigh the sample accurately on an analytical balance.
From the loss in weight of the sample %VM is calculated. This accurate method of determining %VM therefore involves a 5 - 6 h
Determining the volatile matter content o f raw natural rubber
319
procedure with the laboratory responsible for testing a few hundred samples a day requiring a high equipment capability as well as many testing personnel. The test is also carried out for production control purposes in dry rubber manufacturing factories and they find the procedure lengthy, costly and inconvenient. This project attempts to retain the accuracy of the BS test method for %VM determination whilst employing a microwave oven in place of the conventional air circulating oven to reduce the duration of the oven drying time.
EXPERIMENTAL Several samples of TSR made out of both latex and scrap rubber in Sri Lanka, and containing low, medium and high levels of moisture, were selected for this test. From these samples two identical sets were taken for the %VM determination by both the conventional oven method and the microwave oven method. The test temperature was maintained at 100 + 5 °C in the conventional oven while the microwave oven was set to medium power level (50%/320 W). The details of the ovens used for this experiment are as follows. (1) (2)
Conventional oven--Gallenkamp OVB-305. Ventilated and of the air-circulating type. Microwave oven--Matsui Model No. 165 operating on five power levels. Operating frequency 2450 MHz and output 90650 W.
PROCEDURE Homogenisation, milling and preparation of all samples selected for this test were carried out according to the BS test method No. 5923, Part 3, 1981. Each of the two identical sets of samples were then placed in the two types of oven without touching each other and with identification labels. Samples in the conventional oven were heated for a fixed period of 4 h, but those in the microwave oven were heated for varying periods of time, from 10 to 100 min. For each period of drying in the microwave oven a fresh set of samples was used while the corresponding set of identical samples was dried in the conventional oven for 4 h. Samples removed from each oven were cooled to room temperature in a dessicator and then weighed accurately to four decimal places. The
P. A. D. T. Vimalasiri et al.
320
percentage volatile matter content in each sample was calculated as follows.
%VM content = W1 - W2 x 100
wl
where W~ = Initial Wt of the sample and W2 = Wt after heating.
RESULTS AND DISCUSSION The results in Table 1 and a plot of the difference in %VM (A%VM) between the two methods against time (Fig. 1) show that during the early stages of heating in the microwave oven, evaporation of the volatile components from the rubber sample is incomplete thereby leading to a significant deviation in the %VM measured by the two methods. However, an increase in the heating time leads to a narrowing of the deviation in the results obtained by the two methods and at 90 min of heating the two values are identical. Thus it appears that 90 min of heating in a microwave oven set to a medium power level of 320 W is equivalent to a 4 h heating process in an air-circulating conventional
TABLE 1 Average %VM Values of Rubber Samples Heated in the Microwave Oven for Different Periods of Time and the Corresponding %VM Values Calculated for an Identical Set of Samples by BS Method Time of heating in the microwave oven (min )
Average % VM values recorded
10 15 20 25 35 40 45 75 80 85 90 100
0.48 0-51 0.61 0.56 0-51 0.57 0-38 0-42 0-53 0-41 0-51 0-62
Average % VM by BS method A % VM 0-61 0.69 0.73 0-66 0.64 0.62 0-43 0.47 0.54 0.43 0-51 0-62
0.13 0.18 0-12 0-10 0.13 0.05 0-05 0-05 0.01 0.02 0.00 0-00
Determining the volatile matter content o f raw natural rubber
321
>
© 4J
°u 0 . 1 5
.,-t Ill 0 ©
144
"~ 0. i0
0.05
D
20
40
60
80 Drying
Time
i00
(Min)
Fig. 1. Difference in the %VM contents determined by the conventional oven method and the microwave oven method vs time of drying in the ovens.
oven at 100 + 5 °C for the purpose of moisture evaporation from natural r u b b e r to bring the % V M level to below 0-5% (Fig. 2).
CONCLUSION The d e t e r m i n a t i o n of the volatile m a t t e r content of dry rubber for technical specification of T S R could be accurately and rapidly carried out by a modified British Standard m e t h o d involving the heating of the
322
P. A. D. T. Vimalasiri et al.
O. 70
Q~ v 4J
I 4J 0 00.6
/
/
o 4J 4J
>0.50
......... iI
O. 40
O. 30
20
40
60
80 Drying
IOO
Time(Min.)
Fig. 2. Variation of the %VM contents vs time for rubber samples heated in the two ovens. (A A), %VM using a microwave oven; ( o - - - o ) , %VM using a conventional oven.
Determining the volatile matter content of raw natural rubber
323
test sample for 90 min at the m e d i u m power level in a microwave oven instead of using a conventional oven at 100 °C for 4 h.
ACKNOWLEDGEMENT The authors wish to thank Dr B. Boyd Coorey, the Research and D e v e l o p m e n t Manager, Baxenden Chemical Research Co. Ltd, Lancashire, UK, for helping in many ways while writing this paper.
REFERENCES 1. Tillekeratne, L. M. K. and Vimalasiri, P. A. D. T. (1985). J. Chem. Tech. Biotech., 35B, 117-20. 2. Tyler, S. (1987) J. Malaysian Rubber Res. and Dev. Board, 41](1), 17-20. 3. Heinish, K. F. and Nadarajah, M. (1960). Proc. Nat. Rub. Conf., KL Malaysia, Rubber Research Institute of Malaysia, 893 pp. 4. Tillekeratne, L. M. K., Perera, M. C. S. and Rodrigo, H. V. Effect of fresh water and sea water on different grades of crepe rubber. J. Plast. and Rub. Proc. and Applications (in press). 5. Anon. (1969). International Standards for Quality and Packing of Natural Rubber (The Green Book), Rubber Manufacturing Association, New York. 6. Bristow, G. M. and Sears, A. G. (1985). The effects of water immersion on the quality of raw natural rubber, NR Technol. Q., 16(2), 29-32. 7. Sarathkumara, S. J., Janz, E. R. and Tillekeratne, L. M. K. (1987). J. Chem. Tech. and Biotech., 39, 11-18.