Characterization of curing kinetics and polymerization shrinkage in ceramic-loaded photocurable resins for large area maskless photopolymerization (LAMP)
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Large Area Maskless Photopolymerization (LAMP) is a direct digital manufacturing technology being developed at Georgia Tech to produce ceramic molds for investment casting of turbine airfoils. In LAMP, UV light incident on a spatial light modulator is projected in the form of a structured black and white bitmap image onto a platform supporting slurry comprising a ceramic particle loaded photocurable resin. Curing of the resin is completed rapidly with exposures lasting 20~160ms. Three-dimensional parts are built layer-by-layer by sequentially applying and selectively curing resin layers of 25-100 micron thickness. In LAMP, diacrylate-based ceramic particle-loaded resins with photoinitiators sensitive in the range of spectral characteristics of the UV source form the basis for an ultra-fast photopolymerization reaction. At the start of the reaction, the monomer molecules are separated by van der Waals distance (~10⁴Å). As the reaction proceeds, these monomer molecules form a closely packed network thereby reducing their separation to covalent bond lengths (~ 1 Å). This results in bulk contraction in the cured resin, which accumulates as the part is fabricated layer-by-layer. The degree of shrinkage is a direct measure of the number of covalent bonds formed. Thus, shrinkage in LAMP is characterized by estimating the number of covalent bonds formed during the photopolymerization reaction. Polymerization shrinkage and accompanying stresses developed during photopolymerization of ceramic particle-loaded resins in LAMP can cause deviations from the desired geometry. The extent of deviations depends on the photoinitiator concentration, the filler loading, the degree of monomer conversion, and the operating parameters such as energy dose. An understanding of shrinkage and stresses built up in a part can assist in developing source geometry compensation algorithms and exposure strategies to alleviate these effects. In this thesis, an attempt has been made to understand the curing kinetics of the reaction and its relation to the polymerization shrinkage. Realtime Fourier Transform Infrared Spectroscopy (RTFTIR) is used to determine the conversion of monomers into polymer networks by analyzing the changes in the chemical bonds of the participating species of molecules. The conversion data can further be used to estimate the curing kinetics of the reaction and the relative volumetric shrinkage strain due to polymerization.