(7e) Polylactic Acid-Clay Nanocomposites Via Solid-State Shear Pulverization

Bio-based polymers offer a sustainable alternative to traditional polymers from petroleum derivatives, especially in short lifetime products such as packaging materials. Derived from natural organic resources, these types of polymers degrade under natural conditions in a time period ranging from several weeks to several years. Neat forms of many bio-based polymers, however, exhibit many undesirable physical properties, often associated with the low glass transition and melting temperatures. Incorporating small amounts of nanoscale fillers, such as layered silicates (clay), into the biodegradable polymer matrix can improve mechanical, thermal, and other physical properties to make them applicable in commercial applications. Studies involving nanocomposites of polylactic acid (PLA), a common bio-based, compostable polymer, and montmorillonite clay have shown significant property enhancement when complete exfoliation and good dispersion of the clay sheets are achieved [1,2]. However, common processing methods used to prepare PLA-clay nanocomposites, such as in situ polymerization and melt processing, have practical limitations [1-4].

We herein propose solid-state shear pulverization (SSSP) as an effective processing method to disperse clay-based nanofillers in bio-based polymer nanocomposites. SSSP has proven to successfully compatibilize immiscible polymer blends [5] and exfoliate silicate sheets in more traditional polymer-clay nanocomposites [6,7]. Unlike conventional nanocomposite fabrication techniques that often require chemical modification, solvents, and/or heat, SSSP processes the polymer nanocomposites simply by maintaining the polymer below its glass transition temperature and subjecting the material to high shear forces to promote filler exfoliation and dispersion. Thus nanocomposites prepared via SSSP are not limited by thermodynamics or transport issues. The continuous operation of SSSP allows for facile scale-up for industrial use in applications such as biodegradable packaging materials.

Commercial grade PLA and pristine and organically-modified clay were processed under different SSSP conditions. Electron microscopy and X-ray diffraction were employed to investigate the exfoliation and dispersion of clay sheets. Differential scanning calorimetry and thermogravimetric analysis were employed for thermal characterization. Mechanical properties were measured using static and dynamic mechanical tests, and the barrier properties were obtained from oxygen permeation and water vapor transmission testing. Results from these tests will be used to compare the performance of PLA/clay nanocomposites processed via SSSP to that of neat PLA and PLA/clay nanocomposites processed via traditional twin-screw extrusion.

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