Functionalization of Low Molecular Weight Natural Rubber: Preparation and Application

Abstract

Liquid natural rubber (LNR), derived from the depolymerization of natural rubber (NR) into shorter chains, was produced through oxidative degradation using NaNO2 and H2O2 in the presence of formic acid (HCOOH). The study examined how reagent concentrations, temperature, and reaction duration influenced the number-average molecular weight (Mn) of the product. Findings indicated that higher concentrations of H₂O₂ and HCOOH accelerated the degradation rate, while increased NaNO2 concentration slowed down the reduction of the Mn. Extended reaction times and elevated temperatures led to a lower Mn​. FTIR analysis showed that the LNR had hydroxyl end groups, due to the breaking of NR chains in an acidic environment, while a carboxyl-terminated LNR formed in an alkaline environment. SEM images revealed that the latex particles in LNR were spherical and smaller than those in NR. The study also found that the reaction orders for [H2O2], [HCOOH], and [NaNO2] were 1.58, 0.79, and -0.65, respectively. Additionally, the pre-exponential factor was 1.04´109 M-1.72 t-1 and the activation energy (Ea) was 78.66 kJ/mol. TGA analysis indicated that the thermal stability of the rubber depended on its Mn ​. LNR with functional end groups showed thermal instability, making it a suitable starting material for various applications. The graft emulsion copolymerization of methyl methacrylate (MMA) onto a liquid natural rubber (LNR) backbone was conducted using CHPO/TEPA as redox initiators. This study examines the influence of process variables such as reaction time, initiator concentration, monomer concentration, and reaction temperature on the grafting reaction, followed by a kinetic analysis of the graft copolymers. The kinetics of the graft copolymerization were analyzed over a temperature range of 55 - 70°C to understand the effects of these variables on the overall copolymerization and grafting rates, as well as on grafting efficiency. This research also derived rate equations for the copolymerization and grafting reactions in the form of power functions of the process factors. FTIR, SEM, and TEM analyses confirmed the formation of graft copolymers, where MMA create a fully closed shell layer on the LNR core particles, resulting in core/shell composites. It was concluded that a lower monomer concentration leads to the highest grafting efficiency (%GE) for MMA grafted onto LNR. The kinetic rate study of PMMA/LNR graft copolymerization provided the relationships for the polymerization rate (Rp) and grafting rate (Rg), with equations Rp (mol/L.min) = 9.75×102 (moI-0.07/L-0.07∙ min0.17)∙exp[-19.68 (kJ/mol)/RT][CHPO]0.19[MMA]0.88t-0.83 and Rg (mol/L∙ min) =1.45×1011 (mol-0.17/L-0.17∙min1.13)∙exp[-75.09 (kJ/mol) /RT] [CHPO]0.55[MMA]0.62t0.13

Increasing the concentrations of CHPO and monomer, as well as the reaction temperature, enhances both Rp and Rg, while increasing the reaction time reduces these rates. For PMMA/LNR graft copolymerization, the activation energy for copolymerization (Eap) was 0.98 kJ/mol, and for grafting (Eag), it was 75.09 kJ/mol. The resulting graft product of MMA-g-LNR can be used as a compatibilizer to improve the miscibility of polylactide (PLA) and polypropylene (PP). Polymers of the PLA, PP, and a PLA/PP blend (70:30 wt%) were prepared using an internal mixer and compression molding, with liquid natural rubber-graft-methyl methacrylate (LNR-g-MMA) added at 0.0, 2.5, 5.0, and 10.0 phr as compatibilizers. The impact of LNR-g-MMA content on the blend's morphology, mechanical properties, water absorption, thermal degradation, and lifetime was explored. Scanning electron microscopy (SEM) showed that the PLA/PP blend exhibited phase separation, while the inclusion of LNR-g-MMA led to a more homogenized and refined blend structure. Thus, LNR-g-MMA served as a compatibilizer to enhance miscibility in the PLA/PP blend. The tensile strength, elongation at break, and impact strength of the polymer blends improved, whereas water absorption decreased with higher LNR-g-MMA content. The thermal degradation kinetics were analyzed over a temperature range of 50-800°C with varying heating rates. Results indicated that blends without LNR-g-MMA had better thermal stability than those with LNR-g-MMA, and stability decreased with increasing LNR-g-MMA content. The Ea was determined using the Kissinger-Akahira-Sunose method. PLA had a much lower Ea compared to PP, and incorporating PP into the PLA matrix increased the Ea. Adding LNR-g-MMA to the PLA/PP blend reduced the Ea. The lifetime of the PLA/PP blends decreased with the inclusion of LNR-g-MMA.

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