Thermal-Induced And Thermo-Oxidative Depolymerization Of Natural Rubber And Its Epoxidized Derivatives
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Date
2015-06
Authors
Betar, Bashir
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Abstract
Thermal-induced depolymerization of natural rubber (NR) and its epoxidized derivatives, ENR25 and ENR50 (NR with epoxy content of approximately 25% and 50%, respectively) was conducted at 200 oC under the following conditions: (1) ambient pressure (760 mm Hg) whereby [O2] ≈ 20.95%, (2) reduced pressure (85 mm Hg) whereby [O2] ≈ 20.95%, and (3) ambient pressure (760 mm Hg) but samples were placed inside sealed containers whereby the initial [O2] ≈ 100%. The starting and depolymerized rubbers (isolated at 1, 2, 4, 8, 12 and 16 hr time intervals) were characterized by means of (i) Gel Permeation Chromatography (GPC), (ii) Gel content determination, (iii) Proton Fourier Transformed Nuclear Magnetic Resonance (1H FT-NMR) spectroscopy, (iv) Fourier Transformed Infrared (FT-IR) spectroscopy and (v) Thermo Gravimetric Analysis (TGA). GPC data reveals that NR depolymerized faster than ENR25 which in turn depolymerized faster than ENR50. The average gel content of NR, ENR25 and ENR50 is found to increase with the increase in depolymerization time interval. Higher concentration of oxygen molecules resulted in higher formation of gel in shorter depolymerization time interval. 1H NMR spectral analysis reveals that the epoxy content of depolymerized ENR25 and ENR50 is practically the same as that of the undepolymerized ENR25 and ENR50, respectively. Quantitative analyses of aldehyde protons of the depolymerized rubbers using both FT-NMR and FT-IR techniques were performed as a means to gain further insights on the mechanism of depolymerization of NR and
its epoxidized derivatives. TGA results show insignificant difference in the thermal
stability of the starting and depolymerized rubbers affirming the notion that the
general molecular structures of the aforesaid materials are similar. The
depolymerization of NR, ENR25 and ENR50 is proposed to proceed via thermalinduced
chain scission of the carbon-carbon allylic bond of isoprene or epoxidized
isoprene units. Subsequently, the reaction of the resulting alkyl radicals with oxygen
molecules result in the formation of hydroxyl and aldehyde as end groups. The
formation of gel (insoluble materials) was the result of crosslinking reactions
between polymeric radicals produced after chain scission. Depolymerized ENR50
with w M of 44777 g∙mol-1 was used as an example to successfully prepare a watersoluble
rubber derivative. This innovative product is characterized by means of
variable temperature 1H and 13C NMR spectroscopy with the use of D2O as solvent.
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Chemistry