Enhancing the Matrix–Fiber Interface with a Surfactant Leads to Improved Performance Properties of 3D Printed Composite Materials Containing Cellulose Nanofibrils

Abstract

Cellulose nanofibrils (CNFs) exhibit characteristics that make them a desirable addition to new composite materials. CNFs are usable in a wide variety of applications such as coatings, personal and healthcare products, packaging, and advanced structural materials. They can also help overcome some performance issues with objects 3D printed by stereolithography (SLA) including dimensional instability and poor mechanical properties. However, CNFs are hydrophilic, making their dispersion in hydrophobic resins common to SLA difficult. Therefore, improvement of performance properties will not be fully realized. In this work, we treated TEMPO-oxidized CNFs (TOCNFs) with the hydrochloride salt of lauroyl arginate ethyl ester (LAE·HCl), a cationic surfactant, to investigate how this coating would affect the performance properties of multicomponent uncured SLA resins and subsequently printed objects. We hypothesized this coating would enhance the dispersion of the cellulose nanomaterials when compared to their uncoated counterparts, which would lead to quantifiable differences among the sample groups. We found that the viscosity of a commercial 3D printing resin (0.34 Pa·s at 30 Hz) increased by nearly an order of magnitude upon addition of even 1 wt % uncoated TOCNFs (2.96 Pa·s at 30 Hz). Moreover, the tensile strength (19.9(5) MPa) and modulus (0.65(5) GPa) of objects printed from the commercial resin decreased when adding 4 wt % uncoated TOCNF (12.5(2) MPa and 0.58(8) GPa, respectively). In contrast, resins having 4 wt % TOCNFs coated with LAE were less viscous (1.25 Pa·s at 30 Hz), and objects printed from them had enhanced tensile strength (24.7­(7) MPa) and modulus (0.78(8) GPa) when compared to both the unadulterated resin and that having uncoated TOCNFs. Our findings show the general utility of using a surfactant with cellulose nanomaterials to homogenize multicomponent resins for 3D printing composite materials with enhanced performance properties.