CoRe²

Context

Carbon feedstock for polymer materials is currently mostly fossil, and the monomers are produced with large (fossil) energy consumption. Linked to this, end-of-life (EoL) polymers are often burnt for recovering energy, producing even more CO2. Too few adequate chemistries/technologies are available for exploiting these EoL polymers as feedstocks for monomers with an undiminished product quality, while physical/mechanical recycling is not suitable for all plastic waste streams or leads to inferior product quality with expensive additive use. There is specifically a need to produce (co)monomer feedstocks (i.e., building blocks) from step-growth/condensation EoL polymers with a quality that allows them to compete with, and to substitute for large fractions the current virgin monomers/prepolymers. Only by providing equivalent ‘drop-in’ substitutes, large-scale industrial implementation can be expected. Moreover, building blocks can be upgraded/detoxified, which enables better control over harmful chemicals that otherwise slowly penetrate into the environment. Finally, other contaminating polymers can either be removed more readily or used as an advantage to create novel compositions if they display similar reactive functionalities, which forms a major problem in physical recycling. The proposal targets generic EoL recycling of the most societally relevant condensation polymers: poly-amides/esters/urethanes/carbonates (PA/ET/UR/C).

Scientific goals

1. Solvolytic and Catalytic Depolymerization of Condensation Polymers 

For poly-esters/carbonates, it is well known how to split them using simple alcohols or water, but in many cases too high temperatures are required. In contrast, polyurethanes and especially polyamides are more stable, and their splitting may be severely equilibrium-limited, producing fragments with too high molar mass. Chemical agents need to be introduced for splitting, such as (polyfunctional) alcohols, amines, ammonia, etc. Hence, we aim at a combined solvolytic-catalytic approach for efficient chemical cycling of poly-esters/carbonates/amides/urethanes. If a solvent/reactant is used, it should be used in limited amounts (≤ 1 kg/kg EoL polymer) and it must be recoverable at low energy cost to be recycled to subsequent re-use. Similarly, any catalyst used to decrease process temperature should be re-usable. We will also seek conditions that allow partial splitting to pre-polymers that can be directly re-used in a polymerization step. Typical waste stream polymer combinations (e.g. PLA in PET) will be studied to investigate either selective depolymerization, or to create a reactive mixture for copolymer synthesis. Modelling of depolymerisation kinetics will serve as a tool to control the process (e.g. minimizing side products), in combination with dedicated analysis (e.g. molar mass distribution (MMD) and functional group spectrum temporal evolution). Specific focus will be on the relevance of diffusional limitations due to viscosity effects and possible phase segregation phenomena, considering a leading modeling algorithm available within the consortium. A final boundary condition is that downstream separation and energy consumption should be minimized, e.g. via spontaneous phase separation, or via evaporation of volatile reactants.

2. Transformation to useful building blocks 

In many cases, further transformation is required after chemolysis to up-cycle the splitting products from 1., e.g. carboxamides from ammonolysis of polyamides, low-molecular urethanes from solvolysis of polyurethane, or bisphenols produced from polycarbonates. Therefore, we target to develop 
following, innovative catalytic reactions that are not yet technically feasible at present and could widen the portfolio of our developed recycling chemistry platform: 

1. biscarboxamides to diamines: carboxamides are ammonolysis products of polyamides. Selective hydrogenation will yield diamines, which are much more valuable than diacids;
2. bisphenols to aliphatic diamines: bisphenols could be re-used in polycarbonates, but via hydrogenation and (double) amination, they could be transformed to aliphatic diamines, which are among the most valuable precursors for aliphatic thermoplastic polyurethanes (TPUs).

3. Upgrading recycling-derived resources - functional esters, alcohols, amines 

We will demonstrate the chemical upcycling of post-consumer waste polyester (e.g. single use PET water bottles, but also post-industrial waste, e.g. fibers) to high performance thermoplastics and polymer networks. Monomers and prepolymers from 1. and 2. will be purified if needed and repolymerized under conventional conditions, with the MMD tuned for the intended end-application (e.g. bottles, fibers) (Figure 2). Full bottle-to-bottle circularity is a target with enormous implications in terms of reduced CO2 generation. We will investigate how use of fully depolymerized polymers (i.e. monomers) or partially depolymerized polymers (i.e. prepolymers) influences the polymerization efficiency. In addition, we will investigate structure-property relationships of polymers generated from depolymerization products (monomers and prepolymers, generated in 1.) to reduce purification needs and verify robustness of the technology (contamination can never be fully excluded). As a thermodynamically consistent kinetic model is developed in 1., the polymerization upgrading step can also be automatically supported by polymer reaction engineering tools with focus on optimizing reaction conditions.

Using transesterification to leverage recyclability, we moreover aim to transform post-consumer PET to a diverse range of higher structures by simply substituting the alcohol (i.e. ethylene glycol) with other constituents (Figure 2) or use new monomers generated in 2. In this manner, we aim to produce 
poly(butylene terephthalate) [PBT] in a single stream starting from PET via exchange of ethylene glycol with 1,4-butanediol (Figure 2). The feasibility of this chemical process will be interrogated for a variety of different building blocks, with an initial focus on understanding the chemical process and establishing structure property relationships for a select set of commercially relevant formulas.

CO₂ gains expected

Major step-growth/condensation polymers (PET, PA, PC, PU ...) have a large footprint:

• CO₂ footprint for production ranges from 3 t CO₂ / t product (for PET) up to 7-9 t CO₂ / t (for PA);
• ‘Energetic recycling ‘ (= incineration) adds another ~2.3 t CO₂ / t.

In a circular vision, any incineration footprint would disappear. Production footprint is replaced by the impact of collection, sorting, and especially, the quantity of chemical agents applied, their recyclability, and the energy input. This observation forms the basis for developing sub 1. low-temperature (≤200°C), low mass input technologies (≤ 0.5 kg/kg). 

The scale of condensation polymers produced in Belgium is large, with ~7.5 Mio ton plastics produced in Belgium yearly, of which ~1 Mio ton PU, 0.4 Mio ton PET, and 0.1-0.2 Mio ton PA and PC each. If 4 t CO₂ can be saved per t of polymer product, the potential gain is 6-7 Mio ton CO₂.