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A rapid and accurate method for determining the dry rubber content and total solid content of NR latex
Polymer Testing 8 (1989) 353-358
A Rapid and Accurate Method for Determining the Dry Rubber Content and Total Solid Content of NR Latex L. M . K. T i l l e k e r a t n e , L. K a r u n a n a y a k e , S. W e e r a m a n
P. H . S a r a t h K u r n a r a &
Rubber Research Institute of Sri Lanka, Telawala Road, Ratmalana, Colombo, Sri Lanka (Received 24 April 1989; accepted 23 May 1989)
ABSTRACT The dry rubber content of field latex is determined roughly by means of the hydrometer known as the 'Metrolac', which is an instantaneous method. However, accurate determination of the percentage dry rubber content (%DRC) and the percentage total solid content (%TS) of natural rubber latex is important for the export certification of shipments of latex in the producing countries. Accurate determination of % D R C and % TS are also important for many other purposes such as research experiments in the field of chemical modification of rubber. At present, determinations of % D R C and % TS are carried out by the BS methods which involve heating the latex or coagulum samples for over 6 h in a conventional oven. Hence these methods are not suitable for a factory producing large quantities of centrifuged latex for production control purposes or for export. This paper reports convenient methods of determining % D R C and %TS of latex in lOmin and 25 rain, respectively, with the same accuracy as the BS or ISO methods.
INTRODUCTION Dry r u b b e r content ( D R C ) of freshly tapped natural r u b b e r latex normally lies b e t w e e n 25 and 35%. This variation is mainly a clonal characteristic while it also d e p e n d s on factors like the climatic conditions and on the intensity of tapping. Owing to the bad practices followed by the tappers, such as addition of too much water for dilution 353 Polymer Testing 0142-9418/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
354
L . M . K . Tillekeratne
et al.
prior to straining the D R C of the latex supplied to the factory by the tappers could fall even below the lower limit of 25%. Field latex fed into centrifuging machines for increasing the D R C up to about 60% by weight should have an initial D R C of at least 30% for efficient operation of the machine. For the determination of D R C of the latex fed into the centrifuge and also for the rough estimation of field weight and factory weight of latex processed, the hydrometer graduated in grams of dry rubber per litre of a latex, known as the 'Metrolac', is used. The accuracy of the Metrolac estimation of the D R C of the NR latex depends on the skill of the technician who operates it. Further, there can be slight seasonal changes in the Metrolac value of the D R C compared with the real value estimated by the British Standard or ISO method. Hence this accuracy is not sufficient for accurate D R C determination of the centrifuged latex either for export certification purposes or for laboratory experiments. Total solid content (TS) of latex is also determined in the laboratory mainly for concentrated latex using the BS m e t h o d or the ISO method. The main difference between the D R C and the TS determining methods is that in the first an accurately weighed sample of latex is coagulated with a 2 0 g / d m 3 acetic acid solution and the coagulum is then washed with water to get rid of excess acid and other soluble amino acids in it prior to rolling and drying. In the TS determination an accurately weighed thin film of latex is dried in an oven until there is no further loss in weight and the total weight of the final residue is taken as the total solid content of that latex. Both TS and D R C are expressed as a percentage of the initial weight of the latex used. The British Standard m e t h o d and the ISO m e t h o d of determining TS and D R C of latex, employ an ordinary air-circulating oven for drying and hence the total time taken for the tests exceeds 6 h. Furthermore since heating is done at 70 + 2 °C in the presence of air, the chance of parts of the rubber undergoing oxidation is also high. The long duration of 6 - 8 h for the performance of the above two tests makes it inconvenient for laboratories handling several samples of latex a day and also for centrifuging factories where this test is carried out for production control purposes. Performance of the test by the present BS/ISO m e t h o d is more costly too. Hence an attempt was made in this project to eliminate the problem of rubber undergoing oxidation during drying for 6 h in air and also to cut down the long duration of the BS/ISO test m e t h o d by employing a microwave oven operating at m e d i u m - l o w power level in place of the conventional air oven.
Rapid analysis of NR latex
355
EXPERIMENTAL Several samples of both field latex and centrifuged latex preserved with ammonia alone or T M T D / Z n O low-ammonia mixture were selected for this test. From these samples two identical sets were taken for both DRC and TS determinations using the microwave technique. Results obtained in each of these cases were cross-checked by the ISO test method using a conventional oven. The test temperature was maintained at 70 + 2 °C in the conventional oven while the microwave oven was set to m e d i u m - l o w power level (50%/320 W). The details of the ovens used for this experiment are as follows. (1) Conventional oven--Gallenkamp OVB-305, ventilated and of the air-circulating type. (2) Microwave oven--National, Model NE-7670 (650 W), operating at seven power levels; operating frequency 2450MHz and output 65-650 W. The acid used was (a) 2% aqueous acetic acid in the normal DRC determination, and (b) 5% acetic acid in ethyl alcohol in the rapid DRC determination. Procedure
The latex was sampled in accordance with one of the methods specified in ISO 123. (a) In the total solids determination, 2 + 0.5 g of latex weighed to the nearest 5 mg was put into a Petri dish of diameter 7 cm and the dish was rotated to spread the latex evenly over the bottom. It was then dried in the microwave oven at the m e d i u m - l o w power level for various times. After heating, the dish with the rubber was cooled in a desiccator and weighed again. (b) In the normal DRC determination, 10 + 0.5 g of latex weighed to the nearest 5 mg was rotated carefully to obtain a uniform thin film on the bottom of the dish. The dish containing latex film was then treated with 50 ml of 2% aqueous acetic acid while it was rotated slowly to make sure that all the latex reacted with acid. The coagulated film of rubber at the bottom of the Petri dish was then washed with distilled water and dried in the microwave oven at m e d i u m - l o w power level for various times and weighed accurately. In the case of the rapid method of DRC determination, a 5% solution of acetic acid in absolute alcohol was used instead of 2% aqueous acetic acid. Washing in this case was carried out with absolute alcohol.
L . M . K . Tillekeratne et al.
356
The rest of the procedure was identical to that in the normal DRC determination method. The percentage DRC or TS in each sample was calculated as follows. % D R C or %TS = (W2/W1) × 100 where W~ = weight of the latex taken; W2 = weight of the dry rubber or the total solid remaining.
RESULTS A N D DISCUSSION From Fig. 1, which is a plot of difference in total solid content determined by the conventional oven method and by the microwave oven method (TS), it is clear that the two %TS values are identical when the microwave heating time is 30 min. Similarly, from Fig. 2 it is clear that the % D R C of latex would be determined by means of a microwave oven operating at the m e d i u m low power level with the same accuracy as the ISO method in 45 min. By the rapid method used in this study where 5% acetic acid in absolute
4.005
3.005 vl i-<1 2.005
1005
0.00
5
15
25 35 Time (mirl)
45
I
55
Fig. 1. Variation of ATS content with time of heating in the two ovens. Time (min)
Average %TS
Differenceof average %TS (ATS)
5 10 15 20 25
65.31 62-31 61-86 61-76 61.65
3.665 0-665 0.215 0.115 0.005
Rapid analysis of NR latex
357
7.0C 60( ~) 5OO U ~ 400 <1
2(30 300' (b(i ii~ II~
0 I0 20 30 Time40 (min)50 f/3 Fig. 2.
Rate of change of A D R C with time of heating in the two ovens.
(a) Normal DRC determination
Time (min) 10 15 25 35 45
= = = = =
5+ 5 10 + 5 20 + 5 30 + 5 40 + 5
Average %DRC
Difference of average %DRC (ADRC)
66-64 63.15 60-815 60-200 59-95
6.740 3-250 0.915 0.300 0.050
(b) rapid DRC determination
Time (min)
Average %DRC
Difference of average %DRC (ADRC)
5 8 10
64.03 60.62 60.29
3.82 0.41 0.08
alcohol was used as the coagulant, the accuracy of the ISO test m e t h o d for D R C determination could be achieved in just 10 min operating at the m e d i u m - h i g h power level. From these results it is clear that by using a microwave o v e n instead of the conventional air-circulating type of oven the D R C and TS content of latex could be determined in 10 min and 25 min respectively with the same accuracy as the BS or ISO methods where the time taken for the test is over 6 h.
358
L . M . K . Tillekeratne et al. REFERENCES
1. Vimalasiri, P. A. D. T., Tillekeratne, L. M. K., Weeraman, S. & Dekumpitiya, A. S., A rapid and accurate method for the determination of VM content of NR. Polym. Testing, 7 (1987) 317. 2. Tillekeratne, L. M. K., Perera, M. C. S. & Rodrigo, H. V., Effect of fresh water and sea water on different grades of crepe rubber. Plastic Rubber Proc. Applications, 8(4), (1987) 245. 3. ISO 126. Rubber Latex-Natural. Determination of DRC, 1982. 4. ISO 124. Rubber L a t e x - D e t e r m i n a t i o n of TS content, 1974.
A Rapid and Accurate Method for Determining the Dry Rubber Content and Total Solid Content of NR Latex L. M . K. T i l l e k e r a t n e , L. K a r u n a n a y a k e , S. W e e r a m a n
P. H . S a r a t h K u r n a r a &
Rubber Research Institute of Sri Lanka, Telawala Road, Ratmalana, Colombo, Sri Lanka (Received 24 April 1989; accepted 23 May 1989)
ABSTRACT The dry rubber content of field latex is determined roughly by means of the hydrometer known as the 'Metrolac', which is an instantaneous method. However, accurate determination of the percentage dry rubber content (%DRC) and the percentage total solid content (%TS) of natural rubber latex is important for the export certification of shipments of latex in the producing countries. Accurate determination of % D R C and % TS are also important for many other purposes such as research experiments in the field of chemical modification of rubber. At present, determinations of % D R C and % TS are carried out by the BS methods which involve heating the latex or coagulum samples for over 6 h in a conventional oven. Hence these methods are not suitable for a factory producing large quantities of centrifuged latex for production control purposes or for export. This paper reports convenient methods of determining % D R C and %TS of latex in lOmin and 25 rain, respectively, with the same accuracy as the BS or ISO methods.
INTRODUCTION Dry r u b b e r content ( D R C ) of freshly tapped natural r u b b e r latex normally lies b e t w e e n 25 and 35%. This variation is mainly a clonal characteristic while it also d e p e n d s on factors like the climatic conditions and on the intensity of tapping. Owing to the bad practices followed by the tappers, such as addition of too much water for dilution 353 Polymer Testing 0142-9418/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Northern Ireland
354
L . M . K . Tillekeratne
et al.
prior to straining the D R C of the latex supplied to the factory by the tappers could fall even below the lower limit of 25%. Field latex fed into centrifuging machines for increasing the D R C up to about 60% by weight should have an initial D R C of at least 30% for efficient operation of the machine. For the determination of D R C of the latex fed into the centrifuge and also for the rough estimation of field weight and factory weight of latex processed, the hydrometer graduated in grams of dry rubber per litre of a latex, known as the 'Metrolac', is used. The accuracy of the Metrolac estimation of the D R C of the NR latex depends on the skill of the technician who operates it. Further, there can be slight seasonal changes in the Metrolac value of the D R C compared with the real value estimated by the British Standard or ISO method. Hence this accuracy is not sufficient for accurate D R C determination of the centrifuged latex either for export certification purposes or for laboratory experiments. Total solid content (TS) of latex is also determined in the laboratory mainly for concentrated latex using the BS m e t h o d or the ISO method. The main difference between the D R C and the TS determining methods is that in the first an accurately weighed sample of latex is coagulated with a 2 0 g / d m 3 acetic acid solution and the coagulum is then washed with water to get rid of excess acid and other soluble amino acids in it prior to rolling and drying. In the TS determination an accurately weighed thin film of latex is dried in an oven until there is no further loss in weight and the total weight of the final residue is taken as the total solid content of that latex. Both TS and D R C are expressed as a percentage of the initial weight of the latex used. The British Standard m e t h o d and the ISO m e t h o d of determining TS and D R C of latex, employ an ordinary air-circulating oven for drying and hence the total time taken for the tests exceeds 6 h. Furthermore since heating is done at 70 + 2 °C in the presence of air, the chance of parts of the rubber undergoing oxidation is also high. The long duration of 6 - 8 h for the performance of the above two tests makes it inconvenient for laboratories handling several samples of latex a day and also for centrifuging factories where this test is carried out for production control purposes. Performance of the test by the present BS/ISO m e t h o d is more costly too. Hence an attempt was made in this project to eliminate the problem of rubber undergoing oxidation during drying for 6 h in air and also to cut down the long duration of the BS/ISO test m e t h o d by employing a microwave oven operating at m e d i u m - l o w power level in place of the conventional air oven.
Rapid analysis of NR latex
355
EXPERIMENTAL Several samples of both field latex and centrifuged latex preserved with ammonia alone or T M T D / Z n O low-ammonia mixture were selected for this test. From these samples two identical sets were taken for both DRC and TS determinations using the microwave technique. Results obtained in each of these cases were cross-checked by the ISO test method using a conventional oven. The test temperature was maintained at 70 + 2 °C in the conventional oven while the microwave oven was set to m e d i u m - l o w power level (50%/320 W). The details of the ovens used for this experiment are as follows. (1) Conventional oven--Gallenkamp OVB-305, ventilated and of the air-circulating type. (2) Microwave oven--National, Model NE-7670 (650 W), operating at seven power levels; operating frequency 2450MHz and output 65-650 W. The acid used was (a) 2% aqueous acetic acid in the normal DRC determination, and (b) 5% acetic acid in ethyl alcohol in the rapid DRC determination. Procedure
The latex was sampled in accordance with one of the methods specified in ISO 123. (a) In the total solids determination, 2 + 0.5 g of latex weighed to the nearest 5 mg was put into a Petri dish of diameter 7 cm and the dish was rotated to spread the latex evenly over the bottom. It was then dried in the microwave oven at the m e d i u m - l o w power level for various times. After heating, the dish with the rubber was cooled in a desiccator and weighed again. (b) In the normal DRC determination, 10 + 0.5 g of latex weighed to the nearest 5 mg was rotated carefully to obtain a uniform thin film on the bottom of the dish. The dish containing latex film was then treated with 50 ml of 2% aqueous acetic acid while it was rotated slowly to make sure that all the latex reacted with acid. The coagulated film of rubber at the bottom of the Petri dish was then washed with distilled water and dried in the microwave oven at m e d i u m - l o w power level for various times and weighed accurately. In the case of the rapid method of DRC determination, a 5% solution of acetic acid in absolute alcohol was used instead of 2% aqueous acetic acid. Washing in this case was carried out with absolute alcohol.
L . M . K . Tillekeratne et al.
356
The rest of the procedure was identical to that in the normal DRC determination method. The percentage DRC or TS in each sample was calculated as follows. % D R C or %TS = (W2/W1) × 100 where W~ = weight of the latex taken; W2 = weight of the dry rubber or the total solid remaining.
RESULTS A N D DISCUSSION From Fig. 1, which is a plot of difference in total solid content determined by the conventional oven method and by the microwave oven method (TS), it is clear that the two %TS values are identical when the microwave heating time is 30 min. Similarly, from Fig. 2 it is clear that the % D R C of latex would be determined by means of a microwave oven operating at the m e d i u m low power level with the same accuracy as the ISO method in 45 min. By the rapid method used in this study where 5% acetic acid in absolute
4.005
3.005 vl i-<1 2.005
1005
0.00
5
15
25 35 Time (mirl)
45
I
55
Fig. 1. Variation of ATS content with time of heating in the two ovens. Time (min)
Average %TS
Differenceof average %TS (ATS)
5 10 15 20 25
65.31 62-31 61-86 61-76 61.65
3.665 0-665 0.215 0.115 0.005
Rapid analysis of NR latex
357
7.0C 60( ~) 5OO U ~ 400 <1
2(30 300' (b(i ii~ II~
0 I0 20 30 Time40 (min)50 f/3 Fig. 2.
Rate of change of A D R C with time of heating in the two ovens.
(a) Normal DRC determination
Time (min) 10 15 25 35 45
= = = = =
5+ 5 10 + 5 20 + 5 30 + 5 40 + 5
Average %DRC
Difference of average %DRC (ADRC)
66-64 63.15 60-815 60-200 59-95
6.740 3-250 0.915 0.300 0.050
(b) rapid DRC determination
Time (min)
Average %DRC
Difference of average %DRC (ADRC)
5 8 10
64.03 60.62 60.29
3.82 0.41 0.08
alcohol was used as the coagulant, the accuracy of the ISO test m e t h o d for D R C determination could be achieved in just 10 min operating at the m e d i u m - h i g h power level. From these results it is clear that by using a microwave o v e n instead of the conventional air-circulating type of oven the D R C and TS content of latex could be determined in 10 min and 25 min respectively with the same accuracy as the BS or ISO methods where the time taken for the test is over 6 h.
358
L . M . K . Tillekeratne et al. REFERENCES
1. Vimalasiri, P. A. D. T., Tillekeratne, L. M. K., Weeraman, S. & Dekumpitiya, A. S., A rapid and accurate method for the determination of VM content of NR. Polym. Testing, 7 (1987) 317. 2. Tillekeratne, L. M. K., Perera, M. C. S. & Rodrigo, H. V., Effect of fresh water and sea water on different grades of crepe rubber. Plastic Rubber Proc. Applications, 8(4), (1987) 245. 3. ISO 126. Rubber Latex-Natural. Determination of DRC, 1982. 4. ISO 124. Rubber L a t e x - D e t e r m i n a t i o n of TS content, 1974.