Oxygen concentration and conversion distributions in a layer-by-layer UV-cured film used as a simplified model of a 3D UV inkjet printing system

- Layer-by-layer lamination and photopolymerization process was analyzed.
- Numerical calculation and experiments of C=C bond conversion agreed.
- Oxygen affects the thickness of unpolymerized layer and conversion distribution.
- The curve of conversion vs. depth shows plateaus.

Three-dimensional (3D) ultraviolet (UV) inkjet printers represent a versatile technology for creating complex functional structures. During their operation, 3D objects are formed by repeating cycles of drawing a UV-curable resin with inkjet nozzles and then solidifying it with UV irradiation. In this study, the activity performed by a 3D UV inkjet printer was simulated by spin casting a 33 µm thick layer of UV-curable resin (containing diurethanedimethacrylate and 1-hydroxycyclohexyl phenyl ketone compounds mixed at a weight ratio of 99:1) onto a Si wafer followed by photopolymerization for 2 s at a UV irradiation of 10 mW cm−2. Afterwards, the second resin layer with a thickness of 33 µm was spun-cast onto the first layer and photopolymerized under the same conditions. The conversion distribution of C=C bonds in the UV-curable resin was investigated via confocal laser Raman microscopy and numerical calculations, which took into account the kinetics of photopolymerization and oxygen inhibition reactions. The confocal laser Raman microscopy technique provided a unique distribution of the C=C bond conversion across the film depth. Thus, the conversion magnitude at a depth of 0 µm was zero and increased to 0.2 at 6 µm. Afterwards, the slope of the conversion distribution plot became moderate until the conversion reached the value of 0.43 at a film depth of 28 µm. Between the film depths of 28 and 38 µm, the conversion remained constant with a variation not exceeding 0.03. After that, the conversion value increased again, reaching the magnitude of 0.48 at a depth of 50 µm and remained constant in the region between 50 and 56 µm (with a variation not exceeding 0.04). At higher depths, the graph slope became moderate again, and the conversion value increased gradually to 0.51 at 66 µm, after which the silicon wafer was reached. As a result, two different plateaus were observed on the conversion distribution plot: between 28 and 38 µm and between 50 and 56 µm (the corresponding conversion variation in these regions was below 0.05). The obtained experimental data were in good agreement with the results of numerical calculations, which attributed the existence of the two plateaus on the plot of the C=C bond conversion distribution to the formation of an oxygen-lean point. In addition, the effects of the UV intensity, irradiation time, lamination time, photoinitiator concentration, and concentration of dissolved oxygen on the oxygen concentration and conversion distributions across the depth direction have been examined. The obtained results revealed that the increases in the UV intensity, irradiation time, and photoinitiator concentration as well as the decrease in the initial dissolved oxygen concentration effectively increased the conversion of C=C bonds in the resin film and decreased the thickness of an unpolymerized layer.

146