Effects of clay type and content on mechanical, water barrier and antimicrobial properties of agar-based nanocomposite films

Brittleness and other properties such as low thermal stability, medium gas barrier properties and low solvent resistance (e.g., against water) of the pure biopolymer film are often insufficient for food packaging applications. The nanoscale distribution of nanoclays (such as montmorillonite, saponite or hectorite), with a high aspect ratio (100-1,500) and extremely high surface-to-volume ratio (700-800 m/g) established significant improvements to the polymer matrix in terms of mechanical, gas barrier, and optical properties at low filler content (less than 5% by weight). Agar-based composite films with different types of nanoclays including Cloisite Na, Cloisite 30B and Cloisite 20A were prepared using a solvent casting method and their tensile, water vapour barrier and antimicrobial properties were tested. Tensile strength (TS), elongation at break (E), and water vapour permeability (WVP) of control agar film were 29.7±1.7 MPa, 45.3±9.6%, and (2.22±0.19)×10 g·m/m·s·Pa, respectively. All the film properties tested including transmittance, tensile properties, WVP, and XRD patterns indicated that Cloisite Na was the most compatible with agar matrix. TS of the nanocomposite films prepared with 5% Cloisite Na increased by 18%, while WVP of the nanocomposite films decreased by 24% through nanoclay compounding. Among the agar/clay nanocomposite films tested, only agar/Cloisite 30B composite film showed a bacteriostatic function against Listeria monocytogenes. The effect of clay concentration tested using Cloisite Na showed a significant decrease in TS and WVP, with increases in clay content. Packaging film properties can be improved by choosing the proper type of nanoclay and its optimum concentration. The increased barrier property against water vapour transmission suggests a great potential of the agar/clay composite films in the application of food packaging.