Terpenes as a sustainable monomer source for 3D printable inks and resins

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Additive manufacturing (AM) is the process by which 3D structures are built layer-by-layer, commonly from polymer based inks, resins or filaments. These techniques generally generate a reduced quantity of waste compared to other manufacturing techniques. AM is particularly useful for the production of complex and custom 3D parts across medical, automotive and aerospace industries. However, as petrochemical sources of monomer deplete and the worldwide quantity of plastic waste continues to increase, it is key that renewably sourced and recyclable alternatives to these inks and resins are pioneered. The field of AM is therefore faced with two fundamental challenges with respect to sustainability, there is a limited range of renewably sourced polymers suitable for AM inks and a reliance on chemical crosslinking between polymer chains to rapidly induce cure hinders recyclability of printed products.The current lack of renewably sourced polymers suitable for AM can be attributed to the rapid reactivities necessary for sufficient print times and the strict viscosity range required of monomers, pre-polymers and printed polymers. Chemical crosslinks are commonly relied upon in AM as a method of rapidly obtaining high molecular weight polymers capable of supporting subsequent layers, crosslinks however tend to be irreversible which presents major challenges for achieving chemical circularity at the end of product life-time. Terpenes and their derivatives offer a particularly promising class of molecular building blocks for renewably sourced polymers. Firstly, terpenes can be sourced from a range of biobased feedstocks, for example -pinene and -pinene are readily obtainable from turpentine an abundant waste oil from the paper and pulp industry. Secondly, a versatile range of polymers can be derived through the functionalisation of terpene skeletons, including polyolefins, poly(meth)acrylates, polyesters and polyacrylamides. Finally, through copolymerisation of "glassy" cyclic terpenes with "rubbery" acyclic terpenes, a range of different materials with varying glass transitions temperature's can be achieved.Living anionic polymerisation (LAP) offers opportunity to synthesise polymers with highly defined mass, dispersity, composition and architecture making it an ideal candidate to produce pre-polymers of highly defined viscosity. Group transfer polymerisation (GTP) displays extremely fast reaction kinetics producing high molecular weight polymers without crosslinks, and therefore presents a compelling alternative to the crosslinking chemistries commonly used across AM.Thus, this project will investigate the LAP of terpene based monomers and the advantages it provides over radical polymerisations, particularly how molecular weight distribution impacts viscosity. The printability of these terpene pre-polymers will then be assessed, leading on to the printing of some novel terpene based formulations through either photoinitiated or two component printing techniques. GTP within a AM technique will also be trailed using terpene based monomers and pre-polymers to produce printed polymers with limited crosslinking, with a particular focus improving the reaction kinetics of some previously slow reacting monomers.