Hydrothermal liquefaction of diverse natural rubber waste streams in subcritical water for bio-oil production and fuel-relevant rheology

Abstract

Hydrothermal liquefaction (HTL) in subcritical water offers a low-severity route to valorize biogenic natural rubber (NR) wastes into liquid fuels and intermediates while suppressing excessive gasification and char formation. Here, four NR feedstocks—Standard Thai Rubber (STR 5 and STR 20), Ribbed Smoked Sheets (RSS), and impurity-rich tapping-line rubber (TLR)—were comparatively assessed to establish process–structure–property relationships across commercial and waste-derived streams. Temperature (285–300 °C), water-to-rubber mass ratio (1:3–2:1), and residence time (10–40 min) were varied in a stirred batch reactor, and products were separated into an organic oligomeric phase, aqueous phase, solid residues, and gases (by mass balance). Optimal conditions (295 °C, 30 min, H₂O/NR = 1:1) yielded up to 93.8 wt% organic oligomers with low solid residue and gas formation. Gel permeation chromatography revealed a pronounced molecular-weight collapse (e.g., STR 5: Mw 7.5 × 10⁵ to 1.9 × 10⁴ g mol⁻¹) with narrowed distributions, which translated into orders-of-magnitude viscosity reduction and a transition from highly entangled elastomers to pumpable oligomeric liquids. Subsequent atmospheric distillation generated a fuel-range distillate with calorific value ∼41.5 MJ kg⁻¹, comparable to commercial diesel, and GC–MS indicated terpene-derived hydrocarbons with limonene as a major component. Notably, aged and impurity-containing feedstocks (RSS and TLR) delivered liquid yields and properties comparable to high-grade STR, demonstrating feedstock flexibility of subcritical HTL for NR waste-to-oil conversion.