Barrier Properties of PLA

The barrier properties of PLA have not been extensively studied. The first articles treating the permeability of PLA film have been published in 1997 [120, 121], when PLA started to be considered for packaging applications. PLA films with various L/D ratios, different crystallinity degree and blends with numerous additives and polymers have been tested in recent years with gases, water vapour and organic compounds. 

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PLA properties are strongly dependent on their molecular weight [10] and stereochemistry, being L- and D-lactic acid content [39]. Indeed a PLLA or PDLA homopolymer can develop a crystalline structure whereas an atactic polymer whose L-lactic acid content is below 93% remains amorphous. Consequently, the polymer structure, crystalline or amorphous form, can be at the origin of modification in the thermal, optical, physical, mechanical, and barrier properties of PLA. 

With the increase in the recent environmental awareness, there is an improved interest from the food industry to replace the existing non-biodegradable thermoplastics with PLA for certain beverage products. PLA bottles are predominantly used for beverage containers, which are not sensitive to oxygen. While barrier properties of PLA bottles may be improved by various technologies, their implementation is currently limited due to higher production costs. 

Permeability is an intrinsic property of a gas-polymer membrane system. Correlations that relate diffusion, solubility, and permeability coefficients of diverse gases in polymers are available. Models and group contribution theories have been developed to predict permeability of gases in polymers. However, general rules or universal correlations are usually not as good at predicting permeability as rules for a particular set of polymers. A detailed description of these correlations can be found elsewhere [29], This section presents the main barrier properties of PLA to O2, CO2 and N2. 

As mentioned previously, the barrier properties of PLA nanocomposites depend both on the particle size and on the dispersion. If the dispersion in two samples is of the same order, the barrier properties depend on the size of the particles and vice versa [118]. Gas and vapor barrier properties of polymer nanocomposite films are increased because of the large aspect ratios of the clays that result in strong interfacial interactions between the polymeric matrix and the nanoclays [123]. Depending on the strength of the polymer/clay interfacial interactions, three broad classes of nanocomposites are thermodynamically achievable (Figure 12.9) [118] ... 

The use of nanoclays to improve the barrier properties of PLA blends was investigated by Plackett and Soder-gard [113], who blended PLA with PCL and added nanoclays. A significant reduction in O2 and water permeability was observed with the incorporation of nanoclays. Furthermore, these authors concluded that it is more difficult to achieve target permeability values using nanoclays if the nanoclays are combined with impact modified PLA. 

The incorporation of cellulose nanostructure into PLA creates a great effect that includes improvement in barrier properties, nucleation effects and foam formation. Sanchez-Garcia et al. [57] have reported the barrier properties of PLA/cellulose nanowhisker composites showing that the addition of 3 wt% of cellulose nanowhiskers into PLA was able to reduce the water and oxygen permeabilities by 82% and 90%, respectively. Cellulose nanowhiskers acted as a shield in PLA and caused the increment of the crystallinity degree, resulting in high barrier properties [57]. 

Giisev and Lusti considered another factor, which is also responsible for the barrier property changes in the local permeability due to the molecular level of transformation in the polymer matrix in the presence of silicate layers. This factor is directly related to the molecular level interaction of polymer matrix with the silicate layers. The PLA/qC16SAP4 is a disordered intercalated system, the favourable interactions between PLA and silicate layers probably due to the formation of phosphonium oxide caused by the reaction between the hydroxy edge group of PLA and alkylphosphonium cation. As a result, the barrier property of PLA/qC16SAP4 is much higher compared to that of other systems. 

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