Constitutive modeling of photopolymerization and its application to 3D printing
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Photopolymerization is widely used in many engineering applications such as coating, dental restoration and 3D printing. It is a complex chemical and physical process, through which a liquid monomer solution is rapidly converted to a solid polymer. Many photo curable materials shrink during the photopolymerization process due to the chemical reaction volume shrinkage, which can induce the residual stress in the photocured parts. The residual stress can cause the distortion and warpage of the photocured parts, which affects the accuracy and functionality of the 3D printed structures. Therefore the constitutive modeling for the photopolymerization is important for it applications. In this research, the free-radical photopolymerization reaction kinetics and material property evolution during the curing process are investigated by experiments and theoretical modeling. The model can be applied to investigate the internal stress development of the photocured parts caused by the volume shrinkage during photopolymerization. The constitutive model was utilized to simulate the material property evolution and mechanics during the digital light processing (DLP) 3D printing process. Through FEM simulation, we calculated the print through error, volume shrinkage induced distortion of the DLP printed structure and the results are compared with the experiments. In addition, a simplified theoretical model was built for the DLP grayscale 4D printing. Structures and devices with reversible shape change (RSC) are highly desirable in many applications such as mechanical actuators, soft robotics and artificial muscles. The grayscale pattern was used to control the light intensity distribution of the UV projector in a digital light processing (DLP) 3D printer such that the same photo irradiation time leads to different curing degrees and thus different crosslinking densities at different locations in the polymer during 3D printing. After leaching the uncured oligomers inside the loosely cross-linked network, bending deformation could be induced due to the volume shrinkage. The bending deformation was reversed if the bent structure absorbed acetone and swelled. Using this 4D printing method, we designed and created RSC structures. The desolvation based 4D printing was studied using experiments and our constitutive modeling. In summary, this dissertation presents the constitutive modeling of the material property evolution and mechanics in free radical photopolymerization and its applications to 3D printing and 4D printing. Compared to previous research on photopolymerization mechanics model, the model in this dissertation considers the detailed material property evolution process, the non-linear viscoelastic property of the material, and the coupling of the material property evolution and mechanical deformation. Therefore it can provide better understanding of the complex material property evolution and mechanical deformation coupling process.