Rheology guiding the design and printability of aqueous colloidal composites for additive manufacturing

Abstract

Vat photopolymerization (VP) and direct ink write (DIW) additive manufacturing (AM) provide complex geometries with precise spatial control employing a vast array of photo‐reactive polymeric systems. Although VP is recognized for superior resolution and surface finish, DIW provides versatility for higher viscosity systems. However, each AM platform presents specific rheological requirements that are essential for successful 3D printing. First, viscosity requirements constrain VP polymeric materials to viscosities below 10 Pa s. Thus, this requirement presents a challenging paradox that must be overcome to attain the physical performance of high molecular weight polymers while maintaining suitable viscosities for VP polymeric materials. Second, the necessary rheological complexity that is required for DIW pastes requires additional rheological measurements to ensure desirable thixotropic behavior. This manuscript describes the importance of rheological measurements when designing polymeric latexes for AM. Latexes effectively decouple the dependency of viscosity on molecular weight, thus enabling high molecular weight polymers with low viscosities. Photo‐crosslinking of water‐soluble monomers and telechelic oligomeric diacrylates in the presence of the latex enables the fabrication of a scaffold, which is restricted to the continuous aqueous phase and effectively surrounds the latex nanoparticles enabling the printing of otherwise inaccessible high molecular weight polymers. Rheological testing, including both steady and oscillatory shear experiments, provides insights into system properties and provides predictability for successful printing. This perspective article aims to provide an understanding of both chemical functionality (photo‐ and thermal‐reactivity) and rheological response and their importance for the successful design and evaluation of VP and DIW processable latex formulations.

The role of additives in rheology and processing

“…Bean, Rau, Williams, and Long give their practical perspective for this related to aqueous colloidal materials (latexes) for vat polymerization and direct ink write processes. [3] What can nanocellulose do in fluids like bioinks, cementitious fluids, drilling fluids, and so forth? The review by Rana and Thakur explains that nanocellulose in these suspensions can impart shear thinning behavior, increase viscosity, stabilize emulsions, retain water, and so forth depending on the characteristics of the nanocellulose used and the type of fluid.…”

Waterborne Polyurethane Latexes for Vat Photopolymerization

“…Vat photopolymerization (VP), also termed stereolithography (SLA) in earlier literature, is a widely utilized additive manufacturing (AM) platform that enables 3D printing of intricate geometries not achievable with legacy polymer processing techniques. The resulting three-dimensional (3D) objects demonstrate isotropic properties, smooth surface finish, and microscale resolution; however, viscosity limitations typically restrict material options to lower molecular weight precursors. − Predominantly, VP employs patterned UV light to selectively cure photosensitive polymers in a layer-by-layer fashion with free-radical photoinitiated polymerization. Upon UV exposure, efficacious VP printing requires a sufficiently high plateau modulus, typically on the order of 10 4 to 10 6 Pa, to maintain geometric fidelity during the printing operation. − The combination of low viscosity limitations and high modulus requirements often provides mechanically poor 3D-printed parts due to high cross-link densities.…”

“…WPUs are commonly employed as coatings or adhesives due to their low viscosities, which enable excellent adhesion and facile processing. As previously mentioned, one drawback of VP is a viscosity limitation (≤10 Pa·s) with traditional recoating processes and mass diffusion limitations between photocuring steps. ,, Colloidal polymers, which are composed of high-molecular-weight polymers, mitigate the requirement of using low-molecular-weight polymeric precursors. Biphasic dispersions decouple the viscosity–molecular weight relationship as high-molecular-weight polymers are sequestered into a discrete nanoscale phase, thus limiting intermolecular interactions and entanglements. , Low viscosity biphasic colloidal polymers enable AM of WPUs.…”