Experimental investigation of a new type of interfacial instability in a reactive coextrusion process

Effects of gas-assisted technology on polymer micro coextrusion

In this paper, a three-dimensional viscoelastic flow model of double-layer micro coextrusion with circular cross-section was established. The simulations were carried out by the finite element method and the influence of wall slip coefficient on micro coextrusion was studied. The results show that gas-assisted technology is suitable for polymer micro coextrusion forming. The differences between the gas-assisted and conventional micro coextrusion process were compared by analyzing the distribution of melts velocity, pressure, shear rate and first normal stress difference. The research results show that in the conventional micro coextrusion process there are pressure drop, shear rate, first normal stress difference and secondary flow. And there is a gradient distribution of melts velocity. The die swell and deformation vary with the forming process parameters, and it is difficult to control the product quality. But in the gas-assisted micro coextrusion process, there is no pressure drop, shear rate, first normal stress difference and secondary flow. The velocity of polymer melts is evenly distributed and the melts are extruded in a plunger shape. The melts do not swell and deform and it is independent of forming process parameters. It is easy to ensure the products quality and the inherent problems of polymer conventional micro coextrusion are well solved.

A Journey from Processing to Recycling of Multilayer Waste Films: A Review of Main Challenges and Prospects

In a circular economy context with the dual problems of depletion of natural resources and the environmental impact of a growing volume of wastes, it is of great importance to focus on the recycling process of multilayered plastic films. This review is dedicated first to the general concepts and summary of plastic waste management in general, making emphasis on the multilayer films recycling process. Then, in the second part, the focus is dealing with multilayer films manufacturing process, including the most common materials used for agricultural applications, their processing, and the challenges of their recycling, recyclability, and reuse. Hitherto, some prospects are discussed from eco-design to mechanical or chemical recycling approaches.

In Situ Detection of Interfacial Flow Instabilities in Polymer Co-Extrusion Using Optical Coherence Tomography and Ultrasonic Techniques

Co-extrusion is a widely used processing technique for combining various polymers with different properties into a tailored multilayer product. Individual melt streams are combined in a die to form the desired shape. Under certain conditions, interfacial flow instabilities are observed; however, fundamental knowledge about their onset and about critical conditions in science and industry is scarce. Since reliable identification of interfacial co-extrusion flow instabilities is essential for successful operation, this work presents in situ measurement approaches using a novel co-extrusion demonstrator die, which is fed by two separate melt streams that form a well-controlled two-layer co-extrusion polymer melt flow. An interchangeable cover allows installation of an optical coherence tomography (OCT) sensor and of an ultrasonic (US) measurement system, where the former requires an optical window and the latter good direct coupling with the cover for assessment of the flow situation. The feasibility of both approaches was proven for a material combination that is typically found in multilayer packaging applications. Based on the measurement signals, various parameters are proposed for distinguishing reliably between stable and unstable flow conditions in both measurement systems. The approaches presented are well suited to monitoring for and systematically investigating co-extrusion flow instabilities and, thus, contribute to improving the fundamental knowledge about instability onset and critical conditions.

Multi-Micro/Nanolayer Films Based on Polyolefins: New Approaches from Eco-Design to Recycling

This paper describes a future-oriented approach for the valorization of polyethylene-based multilayer films. The method involves going from eco-design to mechanical recycling of multilayer films via forced assembly coextrusion. The originality of this study consists in limiting the number of constituents, reducing/controlling the thickness of the layers and avoiding the use of tie layers. The ultimate goal is to improve the manufacturing of new products from recycled multilayer materials by simplifying their recyclability. Within this framework, new structures were developed with two polymer systems: polyethylene/polypropylene and polyethylene/polystyrene, with nominal micro- and nanometric thicknesses. Hitherto, the effect of the multi-micro/nanolayer architecture as well as initial morphological and mechanical properties was evaluated. Several recycling processes were investigated, including steps such as: (i) grinding; (ii) monolayer cast film extrusion; or (iii) injection molding with or without an intermediate blending step by twin-screw extrusion. Subsequently, the induced morphological and mechanical properties were investigated depending on the recycling systems and the relationships between the chosen recycling processes or strategies, and structure and property control of the recycled systems was established accordingly. Based on the results obtained, a proof of concept was demonstrated with the eco-design of multi-micro/nanolayer films as a very promising solution for the industrial issues that arise with the valorization of recycled materials.

Interfacial Phenomena in Multi-Micro-/Nanolayered Polymer Coextrusion: A Review of Fundamental and Engineering Aspects

The multilayer coextrusion process is known to be a reliable technique for the continuous fabrication of high-performance micro-/nanolayered polymeric products. Using laminar flow conditions to combine polymer pairs, one can produce multilayer films and composites with a large number of interfaces at the polymer-polymer boundary. Interfacial phenomena, including interlayer diffusion, interlayer reaction, interfacial instabilities, and interfacial geometrical confinement, are always present during multilayer coextrusion depending on the processed polymers. They are critical in defining the microstructural development and resulting macroscopic properties of multilayered products. This paper, therefore, presents a comprehensive review of these interfacial phenomena and illustrates systematically how these phenomena develop and influence the resulting physicochemical properties. This review will promote the understanding of interfacial evolution in the micro-/nanolayer coextrusion process while enabling the better control of the microstructure and end use properties.

Mechanism of Thermal Degradation-Induced Gel Formation in Polyamide 6/Ethylene Vinyl Alcohol Blend Nanocomposites Studied by Time-Resolved Rheology and Hyphenated Thermogravimetric Analyzer Fourier Transform Infrared Spectroscopy Mass Spectroscopy: Synergistic Role of Nanoparticles and Maleic-anhydride-Grafted Polypropylene

In this study, polyamide 6 (PA) is blended with ethylene vinyl alcohol (EVOH) to yield packaging materials with a balance of mechanical and gas barrier properties. However, the formation of gel-like structures in both polymers because of thermal degradation at high temperatures leads to a processing challenge, particularly during thin-gauge film extrusion. To address this challenge, nanoclays are introduced either directly or via a masterbatch of maleic-anhydride-grafted polypropylene to the PA/EVOH blend and time-resolved rheometry is used to study the effect of different modes of nanoclay incorporation on the kinetics of thermo-oxidative degradation of PA/EVOH blend and its nanocomposites. Time-resolved rheometry measurements allow the acquisition of accurate frequency-dependent linear viscoelastic behavior and offer insights into the rate of degradation or gel formation kinetics and cross-link density. The thermal degradation was studied by thermogravimetric analysis coupled with Fourier transform infrared spectroscopy and mass spectroscopy, allowing the prediction of the possible reactions that take place during the rheological property measurements. The results show that when nanoclays are incorporated directly, the oxidative reactions occur faster. In contrast, in the masterbatch method, oxidative degradation is hindered. The difference in the behaviors is shown to lie in the different nanoclay distributions in the blends; in the blends prepared by the masterbatch method, the nanoclays are dispersed at the interface. In conclusion, the masterbatch-containing blend nanocomposite would benefit processing and product development.

Three-dimensional viscoelastic numerical analysis of the effects of gas flow on L-profiled polymers in gas-assisted coextrusion

In this study, polymer gas-assisted coextrusion experiments were performed. The influence of a traditional coextrusion flow zone on the gas groove and the relationship between the gas pressure and the melt flow rate were studied. To determine the effects of the gas flow on gas-assisted coextrusion, a three-dimensional simulation was developed in which the gas layer was considered as an independent flow zone. The influence of the gas pressure, gas layer thickness and melt flow rate on the melts’ profile and the deflection deformation degree (DDD) was studied, and the relationship between the gas pressure, gas layer thickness and melt flow rate was obtained. The numerical results indicated that a traditional coextrusion flow zone in front of a gas-assisted coextrusion flow zone could allow products to avoid a gas groove. The quality of the products could be improved by decreasing the gas pressure and gas layer thickness or increasing the melt flow rate. Additionally, the minimum gas pressure decreased as the gas layer thickness increased and increased as the melt flow rate increased. The numerical results were in good agreement with the experimental results, despite a slight quantitative error. Therefore, reasonably controlling the gas flow condition is key in practical applications of gas-assisted coextrusion, and the effects of the gas layer should be considered in gas-assisted coextrusion simulations.