Modeling and Experiments on Multilayered Barrier Coatings Containing Water-Sorbent Biopolymers.

Environmental regulations and consumer demand are driving the need for packaging materials to reduce or eliminate the use of petroleum-based plastics by replacing them with natural and degradable alternatives. Water flux through a barrier layer is governed by the differential in water activity between a high side (e.g., an aqueous solution) and a low side, such as humid air. We propose that a water-absorbent layer sandwiched between two barrier layers will act as a sponge and locally raise the water activity. It will thereby lower the activity differential across the first barrier layer and, hence, reduce the water flux through the multilayer for a set time period. We present a theoretical model that predicts the water flux through a multilayered structure of two or more barrier layers sandwiching hydrophilic layers that hold water according to an absorption isotherm. We use this model to evaluate the effects of layer thicknesses and distributions, the barrier permeability, the water-holding capacity, and initial water activity of the absorbent layers. We found that the water mass loss rate decreases when the thickness and water-holding capacity of the absorbent layer increase. The greatest reduction in the mass loss through a multilayer was achieved when the absorbent layer had an initial water activity of 0 (fully dried). The relative thicknesses of the barrier layers in the multilayer also have an impact on the water loss rate; thicker barrier layers on the low-activity side of the multilayer are the most effective in reducing the water flux. We have verified the model in an ideal system. We also investigated a multilayer structure of a waterborne emulsion polymer barrier coating and water-absorbing chitosan, which is a deacetylated form of chitin. Here, the chitosan layer offered little benefit in decreasing the water permeability because the layers were not thick enough and the permeability of the waterborne coating on its own is not sufficiently low. With our design concept for multilayer barriers containing absorbent layers and the underpinning theoretical model, we envisage materials systems with enhanced barrier properties while also using less petrochemically derived plastic.