Quasi-equilibrium phase separation in supercritical fluids

This study describes the discovery of a phase separation phenomenon in supercritical fluids (SCFs). An SCF is technically a single-phase fluid with two sub-domains separated by the Widom line. A pseudo phase transition occurs between liquid-like (LL) and gas-like (GL) states, similar to the gas-liquid phase transition across the coexistence line in subcritical fluids. By extending the analogy, we demonstrate that LL-GL phase separation is possible by generating submicron size LL argon droplets in a GL argon SCF. The GL fluid is in a quasi-equilibrium clustered state well above the critical temperature, with a significant increase in cluster formation rate traversing the critical pressure. The prolonged phase separation over an hour is consistent with a model of mass transport mediated by clusters. It provides the insight that clustering is an essential factor in transport and non-equilibrium thermodynamic processes in SCFs.

INFLUENCES OF PROCESS VARIABLES AND SUBCRITICAL FLUIDS ON EPDM-ASSISTED DEVULCANIZATION OF SBR-BASED GROUND TIRE RUBBER

ABSTRACT The devulcanization reaction of SBR-based ground tire rubber (GTR) in GTR/EPDM blend was investigated through a co-rotating twin-screw extruder by increasing the screw rotation speed in the presence of subcritical fluids (water, ethanol, and propanol). The influences of the screw rotation speed, the promoting agents, and temperature were investigated. The results show that subcritical fluid, which is a swelling agent and reaction medium, promotes the devulcanization reaction, increases the selectivity of crosslink breakage, keeps the extrusion material from oxidative degradation, reduces the gel particle size of the devulcanized blends, and significantly improves the mechanical properties of the revulcanizate. Among different subcritical fluids (water, ethanol, propanol), the influence of subcritical ethanol is significantly obvious. When promoting agent 450 is added under the best reaction conditions (ethanol, 200 °C, 2.2 Mpa, and 600 rpm), the tensile strength and elongation at break of the revulcanizate retains 99.6% and 209% of vulcanized SBR (24.0 MPa, 356%), respectively.

Recycling of resin cured IIR-based ground bladder rubber with the assistance of subcritical fluids

In this work, the devulcanization reaction of isobutylene–isoprene rubber (IIR)-based ground bladder rubber (GBR) in GBR/ethylene–propylene–diene monomer (EPDM) blend was investigated through a co-rotating twin-screw extruder. The influences of subcritical fluids (blank sample, water, ethanol, and n-propanol) and temperatures (160°C, 180°C, and 200°C) were investigated. The results confirmed the effectiveness of subcritical fluids in decreasing the gel content. Moreover, gel permeation chromatography analysis demonstrates that the devulcanizing processes with subcritical fluids are more homogeneous, making the molecular weight of sol detached from the devulcanized blend more uniform. Proton nuclear magnetic resonance spectra confirm that the reactivity of devulcanization of subcritical ethanol was the best. The optimal extrusion temperature for devulcanization is 180°C, at which the mechanical properties of the revulcanized IIR/(devulcanized ground bladder rubber [DGBR]/EPDM) blends achieve the best state. When promoting agent alkylphenol polysulfide (450) works with the assistance of subcritical ethanol at the best reaction condition (180°C, 2.0 MPa, and 500 r min−1), the tensile strength and elongation at break of the revulcanizate retain 94.7% and 110.2% of vulcanized IIR (15.91 MPa, 483.62%), respectively.