Breaking Bad: Deagglomerating TiO₂ in 3D Printable Polymer Composites for Photocatalysis in Environmental Media

The fields of contaminant destruction and polymer nanocomposites are converging to immobilize photocatalysts for the degradation of conventional and emerging contaminants. Novel work exploits the design freedom of high-surface-area structures enabled by Additive Manufacturing to produce customizable, high-surface-area infilled structures containing photocatalysts. While investigations of nanoparticle dispersion in polymers for mechanical performance are available, there remains a specific need for environmental application-focused research to understand how processing impacts nanocomposite structure-property relationships for 3D printing in water treatment applications. This study investigated twin screw extrusion process parameters (temperature, screw speed, and number of extrusions) on the dispersion of photocatalytic TiO₂ particles in a 3D printable polylactic acid (PLA) composite, the resulting effects on thermal processing properties, and ultimately whether there were benefits to photocatalytic performance. A Design of Experiments evaluated the aforementioned compounding parameters on the number, size, and location of TiO₂ agglomerates in PLA (≈20% w/w TiO₂). Significant correlations between different TiO₂ dispersion states and thermal processing parameters were revealed. Higher TiO₂ loadings (≈30 w/w) resulted in higher viscosity and modulus and a smaller processing window for reliable 3D printing. However, all printed structures tested demonstrated similar photocatalytic rates (≈0.32 to 0.37 ± 0.02 h^-1). This is attributable to the observation of better dispersed TiO₂ at the surfaces of printer extrudates and the actual printed structures, despite the differences in the initial agglomerate state related to larger agglomerates within the interior of the filament. These results suggest that TiO₂ dispersion and distribution by twin screw extrusion are sufficient to achieve environmentally effective degradation rates if agglomerates are less than approximately 20 μm (an image analysis cutoff) and if these smaller agglomerates remain near the surface of printed structures. When such dispersion states are achieved, additional efforts to break up agglomerates appear nonessential for acceptable photocatalytic performance.