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PLA with nanoclay incorporated

Polylactide/nanoclay films were produced primarily because of the reduced permeability to oxygen and water vapour that could be expected in fully exfoliated films.

Nanoclay films were produced (a) by compounding with nanoclay and (b) by coating the material with multi-layers of nanoclay. Nanoclays compounded into PLA were obtained from Southern Clay Products (Texas, USA) and consisted of and consisted of Cloisites 10A, 15A, 20A, 25A and 30B. For nanoclay coating, alternating, multiple layers of organic acrylic polymers and inorganic clay was used.

Nanoclays were combined in dry form with various polylactides, both "flexibilized" and "un-flexibilized" and extruded into a film using a pilot plant-scale twin-screw extruder. Analysis confirmed that nanoclay addition enhanced the thermal stability of PLA. Nanoclay was compounded with acceptable appearance.

Compounding nanoclay with PLA resulted in a reduction of OTR to 70-99cm3/m2/24h/bar and of WVTR to 34-57g/m2/24h. These results were obtained on "un-flexibilized" PLA films extruded on a laboratory extruder and confirm that nanoclay compounding can reduce the permeability. However, the desired target values for cheese are not within easy reach. It also seems likely that use of nanoclays to reach the target permeability values would be even more difficult when combined with "flexibilized" PLA.

These results are consistent with the observations of other researchers, in that extrusion of fully exfoliated PLA nanocomposites, and hence achievement of the full potential for permeability reduction in PLA films is critically dependent on a combination of factors including processing conditions, extruder characteristics, and selection of the most appropriate type of nanoclay.

Initial testing of nanoclay coating of "flexibilized" PLA showed a significant reduction in OTR from 154cm3/m2/24h/bar for uncoated " flexibilized " PLA-1 to 14-19cm3/m2/24h/bar and 15-19cm3/m2/24h/bar for " flexibilized" PLA coated with 40 and 60 layers, respectively. The number of layers coated on PLA did not affect OTR. Nanoclay coating did not have any pronounced effect on WVTR, which was 74-75g/m2/24h and 83-99g/m2/24h for " flexibilized" PLA coated with 40 and 60 layers, respectively. In general, the adhesion of the nanoclay coating to the PLA surface was insufficient indicated by small spots of visual delaminating. However, adherence aspect may be solved during further optimisation.

Microbiological tests on polylactide films produced on a pilot plant-scale extruder indicated that nanoclays incorporated into the polylactide films might positively influence properties by facilitating release of a cyclodextrin-encapsulated antimicrobial (e.g., AITC) within the films. Although this finding is complicated by the fact that the films used were likely not true nanocomposites, it deserves further attention in terms of testing with fully exfoliated nanocomposite films. If confirmed, it could have significance for food packaging and other applications.

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