Max-DePoly: Energy-efficient recycling of polyamides

The use of industrial plastic products is extensive because of their low pro-duction costs, valuable chemical and physical properties, and their durability in use. After the use phase, however, the majority of the collected plastic residues are typically burned for energy supply or disposed in a landfill. The Max-DePoly project is based on the emerging idea of the closed cycle economy: plastics are manufactured, used and subsequently chemically depolymerized to monomers. This chemical recycling approach to monomers enables complete reuse of the residues to produce high quality polymers.

The Max-DePoly project aims to develop depolymerization processes following the idea of chemical recycling to monomers (CRM). The aim of the research projects is to recover the monomers in such a high yield and purity that the monomers can be repolymerized without any loss of quality in their properties.

The Max-DePoly research project focuses on the depolymerization of polyamides (PA). Due to their extreme high durability and strength, various polyamide categories are used in sportswear, textiles, carpets, lightweight vehicle construction and in 3D printing. The PA polymers are important thermoplastics that are produced by condensation reactions in which the amide bonds (-NH-C=O-) connect the repeating units in the polymer backbone with one another (see Figure 1). Due to the internal hydrogen bonding in the polymer crystal structure, the amide bonds result in exceptional stability for the structure. The Max-DePoly project uses ionic liquids as unconventional solvents for the depolymerization of the PA fibers.

Figure 1: a. schematical overview of the reactions in the depolymerisation of polyamide-12 (PA-12) to monomers laurolactam. b. The electron charge distribution of the laurolactam monomer.

Ionic liquids are low-melting liquid salts with organic cations and suitable anions with an exceptional solvation behavior. The best compositions of ionic liquids with specific solvation properties are sought in order to break the hydrogen bond between the polymer chains and to decompose the crystalline PA structure. The interaction of asymmetric and flexible ions in ionic liquids with very different sizes and shapes (e.g. EMIM·TFSI, Figure 2) plays a role in the interactions between ionic liquid and polymer and facilitates polymer decomposition. Since ionic liquids have very low vapor pressures, liquid-liquid extraction is a promising technical method for separating the ionic liquid and the monomers.

Figure 2: The search for optimal ionic liquid for depolymerisation reaction.

The technical design of an energy-optimized and sustainable recycling process is another goal of the project (see Figure 3). The overall process optimization includes a technically applicable recovery of the purified monomers as well as a smart recycling of the used ionic liquids.

Figure 3: The process optimisation in terms of energy, cost and sustainability goals.

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