The upcycling of recycled polyamide 6 via novel chain extenders

As the use of plastics such as polyamide 6 (PA6) continues to grow with no indication of a decline, there is a pressing need for more effective strategies such as the use of recycled alternatives. However, recycled alternatives such as mechanically recycled polyamide 6 (rPA6) often exhibit inferior mechanical properties and are unable to be reused for their original purpose. As such, this dissertation aims to improve the properties of rPA6. Chain extenders were typically used to improve the properties of polymers like PA6. However, traditional chain extenders (i.e. caprolactam & oxazoline) were ineffective in improving the properties of PA6, compared to maleic anhydride (MAH) & epoxy chain extenders. Synergistic improvement was observed when a combination of traditional chain extenders was used in PA6. The goal of this research is to understand why there is a synergy between some MAH and epoxy chain extenders to improve the mechanical properties of rPA6 through an efficient reactive extrusion process. Before the selection of suitable MAH and epoxy chain extenders for rPA6, it is important to understand the structural changes of PA6 as a result of mechanical recycling. There is currently no agreement on the degradation mechanism of PA6, where the degradation pathway could change depending on the experimental conditions employed, resulting in the presence of new functional groups. Hence, in this study, mechanical recycling of PA6 was simulated using multiple pass extrusion. It was found that the scission of the N-alkylamide bond is the predominant degradation pathway during mechanical recycling, leading to a reduction in molecular weight. Competing chain recombination reactions of the amine-carboxyl chain ends could result in the formation of branched structures. However, there were no significant changes to the chemical structure of PA6 and an absence of new functional groups was observed from both FTIR and NMR. In the second phase of the research, the best performing MAH and epoxy chain extenders were determined. MAH chain extenders with varying molecular structures (e.g. different polymer backbone, molecular weight) and epoxy chain extenders with different functionalities (e.g. di-, tri-, tetra-functional) were selected and reacted with PA6 in an extrusion plastometer. It was found that the molecular structure and functionalities of the MAH and epoxy chain extenders, respectively, have an impact on the chain extension effectiveness of PA6 observed from changes in melt viscosities. MAH chain extenders with a high molecular weight (Mn: 75,000 g/mol) and bulky backbone lower the amine-MAH reactivity in the melt and reduce the chain extension effectiveness. In contrast, a low molecular weight MAH chain extender (Mn: 1000 g/mol) observed a plasticising effect of PA6 instead. Epoxy chain extenders with a higher functionality (i.e. tri-, tetra-functional) coupled with a smaller molecular weight is more effective in the chain extension of PA6 due to its higher mobility and reactivity in the melt. The best performing chain extenders selected are styrene acrylonitrile MAH (SAM) and trifunctional epoxy. Finally, the dual chain extenders study of SAM and trifunctional epoxy with rPA6 was conducted, where the structures, and subsequently, properties of chain extended rPA6, can be modified by sequencing the addition of trifunctional epoxy during reactive extrusion. The carboxyl-epoxy and competing MAH-epoxy reactions contributed to the generation of structures with varying degrees of crosslinking. The epoxy-carboxyl & epoxy-amide reactions were observed to be favoured over the epoxy-amine reaction. Further mechanical tests conducted on samples without crosslinking found that an increase in the impact strength was due to the presence of long chain branches in chain extended rPA6. The combination of SAM chain extended rPA6 (rPA-M) and trifunctional epoxy chain extended rPA6 (rPA-E) resulted in strain hardening phenomena during tensile test. It was proposed that strain hardening was attributed to the realignment of the highly entangled polymer chains. Overall, the selection of the novel MAH and epoxy chain extenders with a suitable reactive extrusion approach was able to synergistically improve the mechanical properties of rPA6, paving the way for potential upcycling of rPA6 in the textile or automotive industries.