lunes, 12 de marzo de 2012

Clay−Chitosan Nanobrick Walls: Completely Renewable Gas Barrier and Flame-Retardant Nanocoatings

Polysaccharides are naturally occurring polymers that are widely available in nature. Of the many types of polysaccharides, chitin is the second most abundant after cellulose. Chitin is extracted from the shells of crustaceans (e.g., lobsters and shrimp) and the exoskeletons of arthropods (e.g., insects). Despite its abundance, unmodified chitin’s usefulness is very limited, because of its poor solubility in most solvents. Chitosan, which is an amino polysaccharide obtained via the alkaline deacetylation of chitin is soluble in acidic aqueous solutions, because of the protonation of its amino groups at pH <6.2.In addition to its solubility, chitosan is biodegradable, biocompatible, and benign. These traits have led to the significant study of chitosan’s use in biomedical applications, such as drug delivery, wound-dressing materials, artificial skin, and blood anticoagulants.
In an effort to create fully renewable and multifunctional assemblies, thin films of chitosan and MMT clay were deposited on polyurethane (PU) foam and polylactic acid (PLA) film. P Thin films prepared via a layer-by-layer (LbL) assembly of renewable materials exhibit exceptional oxygen barrier and flame-retardant properties. Positively charged chitosan (CH), at two different pH levels (pH 3 and pH 6), was paired with anionic montmorillonite (MMT) clay nanoplatelets. Thin-film assemblies prepared with CH at high pH are thicker, because if the low polymer charge density. A 30- bilayer (CH pH 6-MMT) nanocoating (100 nm thick) reduces the oxygen permeability of a 0.5-mm-thick polylactic acid film by four orders of magnitude. This same coating system completely stops the melting of a flexible polyurethane foam, when exposed to direct flame from a butane torch, with just 10 bilayers (30 nm thick). Cone calorimetry confirms that this coated foam exhibited a reduced peak heatrelease rate, by as much as 52%, relative to the uncoated control. These environmentally benign nanocoatings could prove beneficial for new types of food packaging or a replacement for environmentally persistent antiflammable compounds.
The goal of this work was to develop a truly “green” film with flame-retardant and oxygen-barrier characteristics. Films assembled with high-pH or low-pH chitosan (CH) and clay (montmorillonite, MMT) showed linear growth as a function of the number of bilayers deposited. Higher chitosan pH resulted in much thicker assemblies with higher clay loading. An oxygen permeability of <0.03 × 10−16cm3cm/(cm2s Pa) was achieved with 30 bilayers (30 BL) of CH pH 6-MMT(< 100 nm thick). The combination of all of these features it is generally recognized as a safe material, it has high oxygen barriers, and the transparency exhibited by this film makes it an ideal candidate for food and other types of high-performance packaging.
Clay−Chitosan Nanobrick Walls: Completely Renewable Gas Barrier and Flame-Retardant Nanocoatings
Galina Laufer, Christopher Kirkland, Amanda A. Cain, and Jaime C. Grunlan*
Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United State
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