Calcium carbonate is extensively used in different polymeric yarns and threads to increase their strength and flexibility. Manufacturing of polymer bags is one area where yarns with good stability and flexibility are of utmost importance. Therefore, it is important to ascertain the amount of calcium carbonate in polymer because if used in excess it can affect the monolithic property of the yarn.
Brief description of the method
- Amount of CaCO3 in polymeric yarn was estimated using classical acid-base titration using selective indicators in different pH range
- Amount of inorganic carbon and corresponding amount of CaCO3 was estimated using DTA-TGA and the results were compared with the wet chemical method
- Alternatively, the amount of calcium could be determined by igniting the yarns at 900°C, followed by treating the residue with hydrochloric acid and complexometric titration with EDTA.
Polymeric yarns were mixed with HCl in a closed system to generate CO2 equivalent to the amount of CaCO3 present in it. Thereafter, the CO2 was absorbed in excess NaOH solution to generate Na2CO3. A certain volume of this Na2CO3 solution was then titrated with standard HCl using phenolphthalein as indicator (pKin = 9.6). The titre value (p) corresponds to the amount of NaOH and half of the amount of Na2CO3.
In another attempt the same volume of the solution as the previous one was titrated against standard HCl using methyl orange as indicator (pKin= 3.7). The titre value (m) indicates the amount of total NaOH and total Na2CO3. As evident from the chemical reactions involved in the process, the difference in the value 2m-2p is the amount of Na2CO3 present in the solution which is equivalent to the amount of CO2 generated by the polymers. Amount of CaCO3 could easily be calculated as the CO2 produced is equivalent to the amount of CaCO3 present in the polymer.
CaCO3 + 2HCl = CaCl2 + CO2
CO2 + 2NaOH = Na2CO3 + H2O
Reactions involved in titrimetry
Using phenolphthalein as an indicator
NaOH + HCl = NaCl + H2O
Na2CO3 +HCl= NaHCO3 + NaCl
Using methyl orange as an indicator
NaOH + HCl = NaCl + H2O
Na2CO3 + 2HCl = 2NaCl + CO2 + H2O
Thermo-Gravimetric Analysis (TGA) of polymer yarn
The TG analysis was carried out in a Perkin Elmer STA 6000 instrument. Up to 700°C, loss of various components of the polymer was observed that accounted for approximately 88% of the total mass. After that, the loss of CO2 was observed at the temperature range of 700 °C to 950 °C. The amount of CaCO3 so determined was in good agreement with the results of wet chemical analysis.
Table 1: Results obtained from TG analysis:
Table 2: Comparison of results obtained from wet chemical process and TG analysis:
TGA analysis plots of sample A and B
Analysis of calcium carbonate using conventional wet chemical approach was found to be accurate and reliable. The reagents used are inexpensive and the repeatability of the method is quite good. The results are also in good agreement with the TG data. Consequently, in absence of an expensive TG instrument, this method could be an efficient alternative for the determination of CaCO3 present in polymer yarns.
Contributed by: Sukhenjit Datta and Alakta Saha under the guidance of Prof. B.K. Gupta