Experimental Characterization of Photo-Sensitive Polymers to Optimize UV Usage Parameters

The research describes the current experimental work and corresponding theory to characterize the light and heat absorption surface localization effects during stereolithographic curing processes. Stereolithographic curing is a common practice of additive manufacturing utilized to prototype and manufacture complex geometries in a short time frame and at low cost via the crosslinking of liquid polymers under the exposure to UV radiation. Further knowledge of this curing procedure can be utilized by manufacturers to optimize their curing processes. In stereolithographic curing, radiative energy transfer is the primary energy source to initiate crosslinking. The propagating UV light reflects during the curing process which creates an inhomogeneous radiative energy transfer in the sample and in turn a nonuniform variation of mechanical properties within the final cured product. The research intends to characterize this inhomogeneous mechanical property distribution.

The intention of this research is to identify the mechanical behavior effects due to non uniform UV light propagation so that optimal curing parameters can be determined. The current research has identified an in-house crosslinking method for a photosensitive polymer. The photosensitive polymer is a polydimethylsiloxane (PDMS) base with a photoinitiator addition made of benzophenone and xylene to make the polymer sensitive to UV light, in addition to the base already being curable via heat additions. The photo-sensitive polymer is cured via both UV and heat exposure as this is necessary to fully cure the polymer, for UV energy alone is not sufficient. In the initial experimentation the polymer is cured in the shape of an ASTM-D638 type V tensile bar to be used as a standard for quantifying mechanical behavior via tensile testing. This standard is intended to be used as a benchmark for all samples to compare mechanical properties.

To determine the effects of the radiative energy transfer the independent variables of the photoinitiator (benzophenone and xylene mixture), UV exposure and heat exposure will be isolated in individual curing conditions. The effects intended to be answered are the following: How does the UV radiation reflection affect the crosslinking of the sample and the resulting inhomogeneous mechanical properties? How does the inhomogeneous mechanical property distribution affect the fracture response? What, if any, stress concentrations form? What are tuned UV exposure parameters to optimize the mechanical behavior of the polymer?

The preliminary experiment utilizes ASTM-D638 tensile testing to identify the surface localization severity and its effects on mechanical behavior. Using these tensile tests in comparison to the separation of independent variables, I plan to characterize the localization effects of non uniform UV light propagation.