As additive manufacturing (AM) expands as a processing technology for structurally customizable materials, there is increasing interest in printing with high particle contents. For suspensions with particle contents of over 50 vol%, there are significant formulation and processing challenges due to increased interparticle friction and suspension complexity. We focus on suspensions with bimodal particle distributions and two common binder systems, a high molecular weight polymer in a solvent that solidifies via solvent evaporation and a monomer mixture that cures via ultraviolet irradiation. We examine the interplay between formulation and processing and show that the formulation effects are particularly important at optimal printing parameters, but that they are overcome by processing-related defects at sub-optimal parameters. By understanding the processing and formulation effects specific to direct ink write AM of high solids suspensions, new customized inks can be designed for a wide range of applications, including construction, energetics, and ceramics.
Polyelectrolytes are used in paper manufacturing to increase flocculation and water drainage and improve mechanical properties. In this study, we examine the interaction between charged cellulosic nanomaterials and polyelectrolyte complex coacervates of weak polyelectrolytes, polyacrylic acid salt, and polyallylamine hydrochloride. We observe that by changing the order of addition of the polyelectrolytes to cellulose nanofibers (CNFs), we can tune the interactions between the materials, which in turn changes the degree of association of the coacervates to the CNFs and the rate at which they aggregate. Importantly for the papermaking process, when adding the polyelectrolytes sequentially to the CNFs, we found faster aggregation to the fibers and lower water retention values compared to those when preformed coacervates or CNFs by themselves were used. Coarse-grain molecular dynamic simulations further support the fundamental mechanism of aggregation by taking into consideration the interaction between cellulose and the complexes at the molecular level. The simulations corroborate the experimental observations by showing the importance of strong electrostatic interactions in aggregate formation.
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