Progress of application of reactive extrusion in polymerization and modification of nylon

1 Reaction extrusion technology
Extrusion is an increasingly widely used material processing and functional technology, usually using a single or twin screw rotation along the extruder barrel to shear and transport materials. The high temperature and short time process in the extruder, as well as the pressure and shear effects, can make the material fully mixed, while causing a large number of physical modifications and chemical reactions, giving, changing or enhancing the functional properties of the material. At the same time, the unique geometry of screw extruders provides screw extruders with a unique ability to efficiently melt polymers and handle (transport and mix) high viscosity systems, especially for chemical reactions during heat treatment of polymers and/or polymerizable monomers. Because the reaction (i.e. polymerization of monomers or grafting of polymers) usually occurs in the molten state, the use of solvents is not required, the solvent-based synthesis and recovery process steps are avoided, and energy and resource savings are achieved, which is an environmentally friendly technology.

2 Application of reactive extrusion
2.1 Extrusion polymerization

Reactive extrusion polymerization is a strengthening process that maximizes reaction rates at maximum monomer and initiator/catalyst (if available) concentrations. It also allows polymerization to take place at higher temperatures without having to take measures to prevent the solvent from evaporating or working under pressure. ε-caprolactam (CL) can be polymerized by anionic ring-opening polymerization (AROP) in a few minutes with a high conversion rate, making it ideal for reactive extrusion. ω-dodecactam (LL) can also undergo AROP to produce PA12 in the presence of a strong base. Reactive extrusion can be in-situ composite using LL solutions containing elastomers such as ethylene-butyl acrylate copolymer (Lotryl). The impact properties of PA12-Lotryl blends were significantly improved by in-situ polymerization. This polymer system is widely used and can also be used for liquid injection and in situ composite preparation. In addition to monomer polymerization, reactive extrusion can also be applied to the reactive processing of polymers.

2.2 PA6 chain extension branching
Regulating the melt rheology and chain relaxation dynamics of polymers can make the polymers easier to process and more widely used. Molecules in the PA6 chain have active amine and carboxylic acid groups at the end of the chain that can react with other functional groups of organic/inorganic materials. In particular, the anhydride or epoxy group can react with the amine group at the end of PA6 chain, and the rheological properties can be controlled through the chain extension and branching of PA6.

PA11 has high ductility and impact strength as well as better thermomechanical properties compared to polylactic acid (PLA), which makes it a suitable candidate for blending with PLA. However, PLA/PA11 is incompatibility, and PA chain extension is an effective means to improve its compatibility. The relative reactivity of chain extender to PLA and PA11 during extrusion is the key to improve the compatibility of PLA/PA11. Compared with solution blending, this method is more environmentally friendly and less costly.

2.3 Additive blending

Blends of PA with commercial polymers such as polypropylene (PP), polyethylene (PE) and polystyrene (PS) have been studied for many years in order to improve the hygroscopicity, processability and cost reduction of PA. The main difficulty of PA blending with polyolefin is the inherent incompatibility between polymers. Reactive extrusion technology is widely used for polymer blending modification due to its combination of efficient mixing and flexibility of reaction conditions in a continuous process. High shear mixing provides the possibility for micron or even nanoscale compatibilization blends, and PA can even form nanoscale blends with fluoropolymers. Bio-based polymer, especially PLA, is a biodegradable bio-based polymer with high tensile modulus and strength. However, its high brittleness, slow crystallization rate, poor heat resistance, low ductility and impact strength limit its application. Blending PLA with another polymer with complementary properties is an effective and economical way to overcome these shortcomings. The compatibility of PLA with different PA blends determines the main properties of these blends, such as microscopic morphology, thermal properties and mechanical properties.

2.4 PA6 microstructure regulation

Due to the complex flexible chain structure and hydrogen bonding, polymorphism is one of the most important characteristics of PA crystallization behavior. In the process of polymer processing, it is very important to control the microstructure, especially the crystal morphology, to improve the mechanical properties and obtain good thermal properties. The remarkable properties of many biological materials stem from their hierarchical structure and the control over order and disorder at different length scales. The polymer blending process is often accompanied by the development and formation of micro-multiphase systems, and reactive extrusion is an effective way to control the morphology of blends. Using special reactions, reactive extrusion can also achieve the coupling of polymerization and material forming.

2.5nm blend

Carbon-based nanomaterials, such as carbon nanotubes (CNT), graphene, nanodiamond (ND), etc., have excellent surface, mechanical and thermal properties. However, the interfacial adhesion between polymer chains and nanofillers has a great influence on the properties of nanocomposites, while the structure of the original nanomaterials leads to hydrophobicity, chemical inertiness, agglomeration and accumulation, which limits their potential applications. The covalent interaction results in better stability and dispersion of functional carbon-based nanomaterials. Clay as a nanoparticle can also be used to accommodate polymer blends and enhance their structural properties. The introduction of sulfur into the polymer skeleton can give the material special properties.

2.6 Material toughening

Acrylonitrile-butadiene-styrene (ABS) is a thermoplastic polymer material with high strength, good toughness and easy processing. The combination of PA6 and ABS can make use of strengths and avoid weaknesses, and overcome the weaknesses of PA6's poor impact performance and high water absorption. As a green bio-based PA, PA56 has excellent fatigue resistance, impact resistance and long service life, but its toughness is slightly insufficient, and unmodified PA56 is difficult to process.

2.7 Flame retardant modification

Pure PA flame retardant grade is low, such as the vertical combustion of unflame retardant PA6 can only reach UL 94 V-2 level, the limiting oxygen index (LOI) is about 24%, and the combustion process will produce dripping and cause fire. Phosphonate is particularly suitable for PA copolymerization because of its good reactivity, flame retardancy and environmental friendliness.

2.8 Membrane material functionalization

PA6 combined with ethylene-vinyl alcohol copolymer (EVOH) can obtain packaging materials with balanced mechanical properties and gas barrier properties. However, high temperature thermal degradation during film extrusion leads to the formation of gel-like structures in both polymers, which poses a processing challenge.

2.9 Recovery and recycling
Plastic recycling is an important way to solve the problem of white pollution, even more environmentally friendly than the use of bioplastics. Compared with landfills, incineration and chemical treatment, polymer recycling is more economical and environmentally friendly. PA6 is one of the most important and valuable recycled plastics. Reactive extrusion is the most widely used method in the recycling of thermoplastics, with little investment and relatively simple equipment. However, during the melt post-treatment process, high heat and mechanical forces can cause chemical changes and degradation of the polymer structure, affecting the thermal and mechanical properties of the polymer, thus limiting its application field. Chain extension is a relatively simple and inexpensive method in the recycling plastic compounding process.

3 Conclusion
Reactive extrusion technology is widely used for PA polymerization and modification due to its combination of efficient mixing and flexibility of reaction conditions in continuous processes, including chain extension branching, microstructure regulation, volume enhancement blending, material toughening, membrane material functionalization, flame retardant modification, and plastic recycling. Reusing waste plastics into products with higher "value" through reactive extrusion is in line with the new concept of sustainability and upcycling. However, there are some inherent limitations of reactive extrusion that need to be solved: First, the design of reactive extrusion is complex, which not only involves a variety of disciplines, but also needs to consider multi-scale problems from molecular scale, micro scale, mesoscale to macro scale. Second, the conflicts between reaction rate and high throughput, precise temperature control and high viscosity, specificity and universality need to be coordinated. Thirdly, the qualitative and even quantitative relationships among reaction system, screw parameters, extrusion conditions, chemical structure and microscopic morphology need systematic theoretical research and guidance. It is believed that with the deepening of research, including the combination of 3D printing, supercritical fluid, modeling and simulation technologies, reactive extrusion can give full play to the characteristics of fast, continuous and environmentally friendly, and play an increasingly important role in the polymerization and processing modification of plastics such as PA.