Rendering Polyurethane Hydrophilic for Efficient Cellulose Reinforcement in Melt‐Spun Nanocomposite Fibers

Abstract

stiff but also biocompatible, biodegradable, recyclable, and hugely abundant. [3] These rod-shaped nanocrystals have a stiffness similar to Kevlar, [4] which makes them ideal candidates for fabricating reinforced polymer nanocomposites. [5] Despite increasing research efforts, CNC reinforcement is not yet applicable to commercial polymer products. [6] Generally, the incompatibility between the inherently hydrophilic CNCs and the mostly hydrophobic commodity plastics is still challenging, as is the integration of CNCs into common industrial processing.Among the various commodity plastics, polyurethanes (PUs) are particularly interesting candidates for cellulose reinforcement. They are used in a broad range of industrial applications due to their outstanding mechanical properties and versatility. [7] Thermoplastic PUs, for example, find use in the automotive industry as they are tough, flexible, and resistant to abrasion, but they lack mechanical strength and are therefore commonly reinforced with carbon fibers. [8] Aiming at "green" reinforcement alternatives, recent research on PU and other polymer nanocomposites instead focused on CNCs with the specific goal of achieving efficient reinforcement using industrially relevant melt processing techniques. [6] There are currently only few examples of CNC/PU nanocomposites produced by melt processing, [9][10][11][12] or other Many commodity plastics, such as thermoplastic polyurethanes (PUs), require reinforcement for use as commercial products. Cellulose nanocrystals (CNCs) offer a "green" and scalable approach to polymer reinforcement as they are exceptionally stiff, recyclable, and abundant. Unfortunately, achieving efficient CNC reinforcement of PUs with industrial melt processing techniques is difficult, mostly due to the incompatibility of the hydrophobic PU with hydrophilic CNCs, limiting their dispersion. Here, a hydrophilic PU is synthesized to achieve strong reinforcement in melt-processed nanocomposite fibers using filter paper-sourced CNCs. The meltspun fibers, exhibiting smooth surfaces even at high CNC loading (up to 25 wt%) indicating good CNC dispersion, are bench-marked against solventcast films-solvent processing is not scalable but disperses CNCs well and produces strong CNC reinforcement. Mechanical analysis shows the CNC addition stiffens both nanocomposite films and fibers. The stress and strain at break, however, are not significantly affected in films, whereas adding CNCs to fibers increases the stress-at-break while reducing the strain-atbreak. Compared to earlier studies employing a hydrophobic (and stiffer) PU, CNC addition to a hydrophilic PU substantially increases the fiber stiffness and strength. This work therefore suggests that rendering thermoplastics more hydrophilic might pave the way for "greener" polymer composite products using CNCs.