Increasing the Dissolution Rate of Polystyrene Waste in Solvent-Based Recycling

Abstract

Solvent-based recycling of plastic waste is a promising approach for cleaning polymer chains without breaking them. However, the time required to actually dissolve the polymer in a lab environment can take hours. Different factors play a role in polymer dissolution, including temperature, turbulence, and solvent properties. This work provides insights into bottlenecks and opportunities to increase the dissolution rate of polystyrene in solvents. The paper starts with a broad solvent screening in which the dissolution times are compared. Based on the experimental results, a multiple regression model is constructed, which shows that within several solvent properties, the viscosity of the solvent is the major contributor to the dissolution time, followed by the hydrogen, polar, and dispersion bonding (solubility) parameters. These results also indicate that cyclohexene, 2-pentanone, ethylbenzene, and methyl ethyl ketone are solvents that allow fast dissolution. Next, the dissolution kinetics of polystyrene in cyclohexene in a lab-scale reactor and a baffled reactor are investigated. The effects of temperature, particle size, impeller speed, and impeller type were studied. The results show that increased turbulence in a baffled reactor can decrease the dissolution time from 40 to 7 min compared to a lab-scale reactor, indicating the importance of a proper reactor design. The application of a first-order kinetic model confirms that dissolution in a baffled reactor is at least 5-fold faster than that in a lab-scale reactor. Finally, the dissolution kinetics of a real waste sample reveal that, in optimized conditions, full dissolution occurs after 5 min.

Figure 1

Schematic representation of the dissolution process for polymer molecules (non-cross-linked, amorphous, and glassy states). Reprinted with permission from ref (6). Copyright 2014 Royal Society of Chemistry.

Figure 2

Schematic of the lab-scale reactor.

Figure 3

Side view of the reaction vessel (left), impellers (middle), and geometric proportions.

Figure 4

Polystyrene concentrations in various solvents at 50 °C.

Figure 5

The results of the multiple regression model used to predict the concentration of polystyrene dissolved in different solvents at 50 °C.

Figure 6

Dissolution results of polystyrene in cyclohexene at (a) 25 °C, (b) 50 °C, and (c) 75 °C for different particle size fractions.

Figure 7

Dissolution results of polystyrene in cyclohexene at 50 °C with (a) Rushton turbine and (b) propeller.

Figure 8

Examples for the fitting of the first-order kinetic model (eq 2) to the experimental data: (a) dissolution in a lab-scale reactor–for a particle size fraction of 2.0 < d < 2.36 mm at 25 °C, K₁ = 0.0557 min^–1; and (b) dissolution in the baffled reactor: 875 rpm, Rushton turbine, and K₁ = 0.50 min^–1.

Figure 9

The first-order dissolution constant was obtained as a (a) function of the average particle size of the different fractions and temperature (fixed impeller speed of 250 rpm), and (b) function of the impeller speed for the different impeller types (fixed temperature of 50 °C and average particle size of 1.59 mm).

Figure 10

Schematic representation of swelling and polymer diffusion (a) without stirring and (b) under stirring. Adapted from Ghasemi et al. Gray cylinders represent the polymer pellets, and the polymer chains are represented in black.

Figure 11

(a) Dissolution results for waste PS compared with pure PS and (b) dissolved waste in the reactor.

Figure 12

First-order kinetic model (eq 2) fitting to experimental data of waste PS, K₁ = 0.56 min^–1.