Baiping Xu

Low cost, bio‐based, and biodegradable macromolecular thermoplastic starch (TPS) is widely blended with poly(butylene adipate co‐terephthalate) (PBAT) to reduce material cost in agricultural films and food packaging. However, this approach often leads to a significant deterioration in overall performance, especially at higher TPS loading. In this work, PBAT/TPS blends were fabricated using a novel dual‐speed non‐twin screw extruder modified with perturbation ring elements (NTSE‐PRE). The structure and properties of these blends prepared were systematically compared with those prepared by a traditional twin‐screw extruder (TSE). The results demonstrated that the chaotic mixing and tensile force field generated by the PRE contributed to a shorter residence time and narrower residence time distribution, along with enhanced dispersion and distribution efficiency relative to the TSE process. Consequently, the NTSE‐PRE processed blends exhibited a significantly refined TPS dispersed phase and significantly improved interfacial compatibility between TPS and PBAT, which collectively contributed to superior processability and mechanical properties. Specifically, the tensile strength, elongation at break, and tensile toughness of the PRE‐40 were 21.0%, 32.1%, and 39.7% higher than those of TSE‐40, respectively. Moreover, the elongation at break and tensile toughness of the PRE‐60 were 258.6% and 190% higher than those of TSE‐60. This study demonstrates that NTSE‐PRE technology offers a viable industrial route for producing high‐performance PBAT/TPS blends compared with conventional processing methods.

To improve the flow characteristics in the weak-mixing regions of conventional symmetric twin-screw extruders (TSEs), this study proposes asymmetric baffle twin-screw (BTS) elements with different screw baffle heights. The flow and mixing behavior in BTS and conventional twin-screw (TS) configurations under partially filled conditions are simulated using the Implicit Incompressible Smoothed Particle Hydrodynamics method, which provides improved solution accuracy and numerical stability. Visualization experiments confirm good agreement between the simulation results and experimental observations. The BTS elements induce an alternating pattern of fluid splitting and merging in the intermeshing zones, as opposed to the isolated helical flow pattern and symmetric conveying characteristics in conventional TSE elements. This mechanism enabled by the novel BTS configurations significantly improves distributive mixing, with greater geometric asymmetry further amplifying chaotic mixing, across a broad range of fluid viscosities.

The metering section of an industrial‐scale single screw extruder is modeled, and two kinds of discontinuous baffles, three rows of plate baffles, and two types of plow‐shaped baffles, arranged in staggered or parallel ways, are proposed to improve the distributive and dispersive mixing. Considering the narrow gap between the screw and barrel, the finite element method along with the mesh superposition technique is applied to solve fully 3D isothermal flow fields where the fluid is assumed to obey the Carreau constitutive model. The computation codes are successfully developed to achieve particle tracking using the fourth‐order Runge–Kutta scheme. Distributive mixing is evaluated in terms of the evolution of tracer particles and Poincaré sections. Dispersive mixing is then examined in terms of shear rates, mixing index, distribution probability function of mixing index, and its integral function for particles tracking with time. For the same pressure drop and the same screw rotating speed, when compared to the conventional single screw, the numerical simulation results showed that the screw with staggered plow‐shaped baffles achieved better distributive mixing with the output nearly unchanged while three rows of plate baffles significantly enhanced both distributive and dispersive mixing at the cost of output reduction by 13.2%.

The kneading elements play a crucial role in the distributive and dispersive mixing in the conventional co-rotating twin screw extruders. A novel kind of asymmetrical dual-speed co-rotating non-twin kneading elements (NTKE) with a zero staggered angle was designed to introduce symmetry break to improve mixing. Finite element method was applied to solve the time-dependent flow field where mesh superposition technique was used to impose the boundary conditions of moving parts and further to avoid the remeshing at every time steps. The flow and mixing of several fluids obeying the Carreau constitutive model with the different power law exponents were investigated numerically and compared with the conventional co-rotating twin kneading elements (TKE). Distributive mixing was evaluated through evolution of tracer droplets and decaying of variance index with time. Moreover, the dispersive mixing was examined in terms of the statistical distributions of mixing index. An integral function with regard to the mixing index was proposed to evaluate the proportion of tracer particles subjected to the elongation.

Dispersive mixing is an important indicator for conventional co-rotating twin-screw extruders. In this work, a new asymmetric twin-screw extruder was developed to improve dispersive mixing. An aqueous solution of sodium carboxymethyl cellulose (CMC-Na) was employed as a matrix fluid, and the red oil-based ink was used as a tracer. A response surface model was constructed to predict the average diameter of droplets in terms of the feed rate, screw speed, and baffle height, and then the corresponding optimal solutions were obtained. Visualization experiments indicated that the gap regions between the screw and the barrel are mainly responsible for the tracers’ breakup. Particle image velocimetry experiments further confirmed that high velocity appeared in these gap regions and resulted in high shear rates, especially for a baffle height of 15.5 mm, where the linked regions of high velocity turned up.

A new kind of perturbation ring element (PRE) is first proposed to introduce a repetitive topology of splitting and recombination across the intermeshing zones of a co‐rotating non‐twin screw elements (NTSE) with a speed ratio of 2. A numerical simulation is performed using finite element method (FEM) along with the mesh superposition technique (MST). Post‐treatment codes are successfully developed where fourth‐order Runge–Kutta scheme is used to achieve particle tracking. For the tracer particle groups released initially from the upper and bottom intermeshing regions, mixing is characterized in terms of the evolution of tracer particles, mixing variance index, and residence time distribution (RTD). The numerical results revealed for a given output, the larger the screw speed, the larger the dividing ratio, and the better distributive mixing is. PRE achieved the best distributive mixing owing to the changing of flow topology. In TSE there are Komogorov‐Arnold‐Moser (KAM) tubes in which the tracer particles are confined to prevent better mixing from occurring.

A novel kind of dual‐speed corotating non‐twin kneading elements with a zero stagger angle is designed to improve mixing ability. The physical model considering several narrow gaps is developed where the inlet and outlet transition sections are also included. Finite element method associated with the mesh superposition technique is further applied to solve the time‐dependent flow field where fluid is assumed to obey the Carreau constitutive model. The tracers initially from different locations are tracked using a self‐developed fourth‐order Runge–Kutta scheme. Distributive mixing is evaluated through evolution of tracer droplets and decaying of variance index with time. In addition, a Lagrangian statistics method is used to characterize the dispersive mixing in terms of the statistical distributions of mixing index and their integration areas within the range of mixing index larger than 0.5. In contrast, the kneading elements of a conventional twin screw configuration are also modeled to show the mixing difference.