Composite-resin systems for direct digital additive manufacturing

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Direct Digital Additive Manufacturing (DDAM) which is also called as Solid Freeform Fabrication is a production method of making three-dimensional (3D) objects by layer by layer printing. The growth of DDAM is expected to be exponential once it reaches to its turning point. 3D printed structures made of polymeric composites may find broad range of applications such as prototypes, medical and dental products, sporting goods, and spare parts. Fused deposition modelling (FDM), stereolithography (SL) and selective laser sintering (SLS) are current DDAM methods to create parts with controlled architecture and composition. Among them, SL is based on photopolymerization in a curable monomer bath by UV laser beam. SLS is based on inkjet printing and melt fusion of polymer powder on selected areas by infrared laser beam. FDM is based on micro extrusion of thermoplastic molten filaments and their deposition layer by layer. Advantages of SL and SLS are speed and high resolution. The primary advantage of FDM technology is its ability to create almost any shape or geometric feature. On the other hand both SL and SLS generate waste from the cleaning of uncured or unmolten segments. Weak mechanical properties are the disadvantages of all of those methods (SL, SLS and FDM) and limit their applications. Whether cured or fused layers, they have limited formation of interpenetrating polymer networks among polymer particles and layers . Thus they generate weak interparticle (layer) bonding and weld lines. In addition die drool, sag and swell are other problems of FDM which are inherited from conventional plastic extrusion processes. The goal of my program is to address those problems by the discovery of Reactive Extruded Deposition (RED) process. Cellulose nanocrystal (CNC)-polyurethane composites will be used as materials. Selected diisocyanates will be mixed with CNC dispersed polyols and delayed time controlled catalysts in a micromixing chamber. Then, the mixture will be deposited before polymerization starts by the computer controlled micro capillary. Delayed polymerization will take place, once successive layers are deposited and diffusion of molecules among layers takes place. Polyurethanes are selected because of their tunable mechanical and biocompatible properties, suitable for their wide range of applications. On other side, CNC particles will play two roles: First of all, the rheology of the deposition and settling stages will be controlled by CNC particles. Secondly, rod-shaped CNC particles with high modulus of elasticity (134 GPa) will give the desired mechanical properties to finished products at low concentrations with percolation. The program will have five parts: 1) Preparation of monomers; 2) Grafting CNC and dispersing in polyols; 3) Micro mixing and microchannel flow; 4) Rheokinetics of catalytic polymerization and weld formation; 5) Mechanical properties of layered deposition.