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The aim of this study is to find out the viscosity change of urea-formaldehyde (UF) resin with the synthesis parameters namely formaldehyde/urea (F/U) mole ratios, pH and temperature. The viscosity of UF resins, related to molecular weight and activation energy is very important factor of their usability. Urea-formaldehyde (UF) prepolymer was synthesized through polycondensation reaction with F/U ratio 0.8, 1.0, 1.2, 1.4, 1.6. The synthesis was carried out by two steps: alkali catalysis at reaction pH 8.3, 90°C for 60 min and thereafter acid catalysis at pH 4.3, 83°C for 15 min. Viscosity of prepolymer was determined at acid catalysis step by simple glass viscometer. Weight average molecular weight (Mw) was calculated from the viscosity data of UF prepolymer using Mark-Houwink equation. Highest Mw (2020.9) of prepolymer was obtained at F/U molar ratio 1.0 and pH 4.3. In addition, it was found that pH 4.0 yielded greater Mw (2049) UF prepolymer among the four reactions which were performed at pH 4.0, 4.3, 4.7, and 5.0. The energy of activation (Ea) of UF prepolymer was also calculated from the measured viscosity at temperature 70, 75, 80 and 85°C. The highest values of Ea were also found at F/U molar ratio 1.0 and pH 4.0 & lowest values was obtained at F/U molar ratio 1.6 and pH 5.0. From the experimental data, it was shown that the values of Ea and Mw were varied comparably with the change of reaction parameters.

Okra bahmia (Abelmoschus esculentus) plant is considered as one of the abundant sources of natural fibers. Huge amount of okra plant stem is discarded on the field annually after collecting vegetable, without proper utilization. However, this biomass from the okra plant is a renewable, biodegradable, cost efficient and low-density source for production of bast fibers, and other industrial cost-efficient eco-friendly materials. The research on okra bast fiber has started in 2007. After that, the fiber extraction process, composition of fiber, morphology and performance properties of fiber, fiber modification techniques, and some important applications of the fiber etc. have been established. It was found that the okra bast fiber contains high cellulose content, excellent mechanical strength and stiffness, and good thermal resistance which are comparable to some traditional bast fibers like jute, hemp and ramie. Some okra bast fiber reinforced biocomposites were successfully fabricated with different matrices including biodegradable corn starch, Poly(lactic acid), P(vinyl alcohol), urea formaldehyde resin etc. via application of various processing methods. These studies revealed that the okra bast fiber biocomposites exhibited better mechanical properties, water resistance and thermal properties at optimized processing conditions. Therefore, by suitably optimizing the fiber, matrix, processing conditions, the future expectations of the okra bast fibers can be dramatically enhanced and its usage in composite field can be widened. © Springer International Publishing AG 2017.