Abstract
Novel, low density structures which combine biologically-based fibers with clay aerogels are produced in an environmentally benign manner using water as solvent, and no additional processing chemicals. Three different reinforcing fibers, silk, soy silk, and hemp, are evaluated in combination with poly(vinyl alcohol) matrix polymer combined with montmorillonite clay. The mechanical properties of the aerogels are demonstrated to increase with reinforcing fiber length, in each case limited by a critical fiber length, beyond which mechanical properties decline due to maldistribution of filler, and disruption of the aerogel structure. Rather than the classical model for reinforced composite properties, the chemical compatibility of reinforcing fibers with the polymer/clay matrix dominated mechanical performance, along with the tendencies of the fibers to kink under compression.
Highlights
Aerogels are materials that are produced from wet gels by replacement of solvent with air, typically exhibiting bulk densities of 0.1 g/cm3 or lower
In a preliminary study we reported that the combination of biologically-based fibers, domestic clay, and polymers can produce environmentally-benign materials with substantially improved mechanical properties such that they are suitable for a wide range of end uses [25]
Freeze drying of aqueous clay/PVOH/fiber samples readily produced low density, fiber-reinforced aerogel composite materials, as we have reported previously [25]
Summary
Aerogels are materials that are produced from wet gels by replacement of solvent with air, typically exhibiting bulk densities of 0.1 g/cm or lower. We have demonstrated a robust freeze-drying process for converting common smectite clays, such as sodium montmorillonite and bentonite, into aerogels with bulk densities in the range of 0.02–0.10 g/cm3 [9] These clay aerogel materials are fragile, exhibiting mechanical properties similar to those of balls of cotton fiber, and are crushed or irreversibly damaged under low stress levels. Further physical modifications which can substantially enhance the physical properties of polymer/clay aerogel composites include biomimetic mineralization [21], incorporation of rigid nanowhiskers [22], and reinforcement with fibers [23] These aerogels can be incorporated into traditional, fabric-reinforced composite structures, benefiting from adhesive penetration of both fabric and aerogel to produce materials whose mechanical properties and densities are reminiscent of balsa or cork [24]. Exploration of the effects of reinforcing fiber length and composition on mechanical properties is reported in the present work
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