EXPERIMENT 1: Determination of total solid contents (TSC) of NR and NBR latex 1.0
Objective
The TSC of NR and NBR latex according to ISO standard was determined 2.0
Introduction In the chemical industry nomenclature, the term “latex” applies to any emulsion of
polymers, including synthetic rubbers and plastics. “Natural rubber latex” refers specifically to products derived from the milky fluid, or latex, produced by the laticifers of the tropical rubber tree Hevea branziliensis (Bernstein David et al., 1999). They comprise proteins, carbohydrates, lipids and inorganic constituents and represent the main composition difference between natural rubber (NR) and its synthetic counterpart, namely poly (cis-1,4-isoprene) synthetic rubber (IR) (Rainer Hofer, 2009). Nitrile butadiene rubber is a family of unsaturated copolymers of 2-propenenitrile and various butadiene monomers (1,2-butadiene and 1,3-butadiene).
Although its physical and
chemical properties vary depending on the polymer’s composition of nitrile, this form of synthetic rubber is generally resistant to oil, fuel, and other chemicals (the more nitrile within the polymer, the higher the resistance to oils but the lower the flexibility of the material). The total solid content of latex is the percentage by mass of the whole, which is nonvolatile at 105oC for 2 hours in an open atmosphere oven. A quantity of latex is weighed in a flat-bottomed glash dish and dried in an oven at 105oC ± 5oC to the constant weight in 2 hours, and the remaining polymer film is weighed again to find the quantity of the weight of the residue (H. Yildirim Erbil, 2000). Fresh natural rubber latex contains about 30-40% of rubber hydrocarbon that is normally referred to as dry rubber content (DRC). However, the total solid content (TSC) is higher than the DRC due to the presence of non-rubbers in the latex, at around 5%. The DRC and nonrubber content may change due to many factors such as clone, soil and climate conditions, season, type of fertilizers used, and tapping frequency.
Most of these non-rubbers are
dissolved or suspended in the aqueous serum or adsorbed on the surface of rubber particles. They become trapped, tenaciously held, or co-precipitated during coagulation of the rubber probably due to their poor solubility in the aqueous medium or strong entanglement with the rubber molecule (Sabu Thomas et al., 2014). 1
3.0
Material
Natural rubber (NR) latex and Acrylonitrile butadiene rubber (NBR) latex
4.0
Equipment
4.1
Flat-bottomed dishes, lipless, of diameter approximately 60mm, provided with covers
4.2
Oven, capable of being maintained at 70oC ± 2oC or 105oC ± 5oC
4.3
Oven, capable of being maintained at 125oC ± 2oC and at pressure below 20kPa
4.4
Analytical balance, capable of being read to 0.1mg
5.0
Experimental procedure 1. A dish together with its cover was weighed to the nearest 1mg 2. 2.0 ± 0.5g of latex was poured into the dish, the cover was replaced and it was weighed to the nearest 1mg 3. The content of the dish was swirled gently to ensure that the latex covered the bottom 4. 20g of distilled water was added and mixed well with the latex by swirled 5. The dish was placed and uncovered in the oven so that it was horizontal, and it was heated at 70 oC ± 2 oC or 105 oC ± 5 oC until the sample had loss its whiteness or for 15h or 2h respectively 6. It was allowed to cool to ambient temperature in a desiccators replaced the cover and weighed 7. The dish was returned, uncovered to the oven for 80min if drying temperature is 70 oC ± 2oC or for 15min if the drying temperature was 105 oC ± 5 oC 8. It was allowed to cool at ambient temperature in a desiccators replaced the covers and weighed 9. The drying procedure was repeated at intervals of 30min or 15min, until the loss in mass between two successive weighing was loss than 1mg 10. If the sheet became successively sticky after heating at 105 oC ± 5oC and it was suspected that significant oxidation has occurred, repeated the determination at 70 oC ± 2oC
6.0
Results Table 1: Total solids content (TSC) for NR latex and NBR latex. 2
Types of latex
NBR Latex
NR Latex
Samples
Dish A
Dish B
Dish C
Dish A
Dish B
Dish C
Latex Weight (g)
1.7572
1.9448
2.0592
1.6380
1.6280
1.9481
Latex Weight After Oven (g)
0.7993
0.9300
0.9348
1.0361
1.0304
1.2141
Total Solid Content (%)
45.49
47.82
45.40
63.25
63.29
62.32
Average TSC (%)
46.24
70 61
62.95
62.95
60 50
45 46.42
40
Total Solids Content (%)
30
Actual Experimental
20 10 0 NR
NBR
Type of latex
Figure 1: The actual and experimental TSC value for NR and NBR latex
7.0
Discussion Upon drying, latex is transformed from milky, colloidal dispersions to transparent,
continuous films in which the contours of individual coalesced particles can still be discerned. 3
Upon ageing at room temperature, however, these latex films undergo a further gradual coalescence in which the particle contours gradually disappear and any incompatible substance are exuded to the surface (E.B. Bradford et al., 1973). Based on Table 1, the weight of latex for each dishes are different. This is due to the different dimension of dishes used. For natural rubber (NR) latex, dish B showed highest total solid content (TSC) value (63.29%) followed by dish A (63.25%) and dish C (62.32%). For acrylonitrile butadiene rubber (NBR) latex, dish C showed the lowest TSC value which is 45.40% followed by dish A and dish B where the TSC values are 45.49% and 47.82% respectively. Dish A in NBR latex and dish C in NR latex is the biggest dish but showed low TSC value. Theoretically, the bigger surface area of dish, the easier for the volatile material to escape from the latex thus lowest the value of TSC. The comparison between NBR latex and NR latex is the total average of TSC value. NR latex showed higher TSC value (62.95%) and NBR latex showed lower TSC value (46.24%). This is because there a lot of substance in NR latex such as rubber substance, proteinaceous substance, resinous substance, ash, sugar and water. While NBR latex only contains a few substances such as rubber substance and water resulting in lower TSC value. From Figure 1, it showed the theoretical TSC value and experimental TSC value for each NR latex and NBR latex. The experimental TSC value for NR latex and NBR latex are higher compare the theoretical value. The theoretical TSC value of NR latex is 61% compare to the experimental TSC value which is 62.95%. While the theoretical value of NBR latex is 45% and the experimental TSC value is 46.42%. These differences might due to the long term preservation of latex. The latex used in this experiment is preserved with ammonia. The ammonia is easy to evaporate with times thus increase the formation of volatile fatty acid (VFA) resulting increasing in TSC value. During the experiment, some errors may occur and make the value of TSC quite different from the TSC value given.
8.0
Conclusion The surface area of dishes used and type of latex used are strongly influenced the total
solid content (TSC) value. Dishes with greater surface area provide better medium for volatile 4
material to escape during the heating process which resulting in lower TSC value. Natural rubber (NR) latex showed higher TSC value compare to acrylonitrile butadiene rubber (NBR) latex because the present of variety substance in NR latex. The latex used must be preserved with good preservation substance to achieve and maintain better quality of the latex product. The International Standard ISO must be followed thoroughly to obtain accurate results.
9.0
Reference 1. E.B. Bradford and W. K. Carrington. (1973). The Transport of Water Through Latex Films. Symposium No 41, 155-174. 2. H. Yildirirm Erbil. (2000). Vinyl Acetate Emulsion Polymerization and Copolymerization with Acrylic Monomers. United States of America: CRC Press LL. 3. I. Leonard Bernstein, Moiraa Chan-Yeung, Jean-Luc Malo and David I. Bernstein. (1999). Asthma In The Workplace Second Edition, Revised and Expanded. New York: Marcel Dekker, Inc. 4. Rainer Hofer. (2009). Sustainable Solutions for Modern Economics. UK: RSCPublishing. 5. Sabu Thomas, Chin Han Chan, Laly A. Pothen, Rajisha K.R. and Hanna J. Maria. (2014). Natural Rubber Materials Volume 1: Blends and IPNs. UK: RSCPublishing. 6. Subana Shanmuganathan, Sandhya Samarasinghe. (2016). Artificial Neural Network Modelling. Switzerland: Springer.
5
