Advanced toughening concepts for 3D-printable photopolymers

Additive manufacturing technologies (AMT) have gained a lot of interest as potential method for future tool-less manufacturing. The key challenge is to finally print parts whose geometrical as well as mechanical and functional properties are at least as good as those of conventionally (e.g. by polymer injection moulding) manufactured parts. The current dilemma of AMT is the fact that none of the currently available technologies can provide high geometrical quality (surface roughness, precision) and high mechanical qualities (strength, toughness, heat deflection temperature) at the same time. Lithography-based AMT (e.g. stereolithography) have the capability to achieve excellent feature resolution, surface quality and precision, but suffer severely from the fact that the available photopolymers exhibit low toughness and/or low heat deflection temperature. The goal of this project is to provide a new class of thermoplast-like photopolymers which allow to 3D-print parts with high resolution and precision, and at the same time significantly improved thermomechanical properties. The hypothesis, which will facilitate these improvements compared to the state of the art, assumes that a strongly covalently dominated polymer network will always be brittle, since chemical bonds can only be broken irreversibly. A polymer, which is more dominated by physical bonds (secondary bonds between polymer chains) has a larger potential for high toughness, since secondary bonds can be broken reversibly and therefore allow the dissipation of large amounts of energy due to movement of chains against each other. Strong secondary bonds lead to a high viscosity of the monomers, and such high viscosity monomers cannot be processed with currently available lithography- based 3D-printers. By modifying 3D-printers and at the same time developing monomer systems with increased intermolecular forces, this project will experimentally evaluate the above mentioned hypothesis. A heatable vat (60-80C) allows the processing of high molecular weight monomers (~ 5kDa) which in turn ensures low degree of shrinkage. The selected monomers are mono-functional that yield linear or slightly branched polymers with high toughness. Heating above the final glass transition temperature (Tg) of the material ensures high double bond conversion (>98%). The use of methacrylate based monomers ensures sufficient stability during processing. If the mechanical properties are not sufficient due to the influence of the newly formed polymer backbone, chain transfer agents will be used. A degree of polymerization of ~10 is sufficient as similar commercially available thermoplastic urethanelastomers have molecular weights of 50kD. The second critical point of monofunctional monomers is their photoreactivity due to their late gel points. Temporal crosslinks will circumvent this issue.

Additive manufacturing technologies (AMT) have gained a lot of interest as potential method for future tool-less manufacturing. The key challenge is to finally print parts with precise geometry as well as good mechanical properties, which are at least as good as those of conventionally (e.g. by polymer injection moulding) manufactured parts. The current dilemma of AMT is the fact that none of the currently available technologies can provide high geometrical (surface roughness, precision) and high mechanical qualities (strength, toughness, heat deflection temperature) at the same time. Lithography-based AMT shows the potential to achieve these goals, but suffers from inaccessibility of suitable photopolymer resins. The aim of this project was to introduce new photopolymerisable monomer systems for lithography-based AMT, which allow high resolution 3D-printing while providing also good thermo-mechanical properties like strength, toughness and heat deflection temperature. Furthermore, currently available lithography-based 3D-printers were developed to enable processability of this new resins. Therefore, a heatable vat and a heatable coating mechanism were constructed, which also allow processing of highly viscous resins. With this new improved 3D-printers several approaches for toughening of photopolymers were investigated. The tested concepts are ranging from mimicking ABS (a benchmark engineering polymer regarding to toughness), adding different additives to commercially available resins to also testing completely new synthesised monomers. For biomedical applications common photopolymerisable monomers are not suitable due to cytotoxicity and low biocompatibility. Hence, formulations based on biocompatible vinyl esters were developed. Thus, lithography-based AMT of some tough biocompatible materials for tissue engineering could be realized also.