Chilean researchers explore novel approach to eco-friendly food packaging
Polylactic acid (PLA) is a polymer made from renewable biomass; typically, fermented plant starches, such as corn, sugarcane, cassava, or sugar beet. In use, it exhibits similar mechanical and physical properties to some petroleum-derived polymers, but it is biodegradable – under commercial composting conditions, PLA will break down within twelve weeks, compared to centuries for polyethylene terephthalate (PET). Widely used in the biomedical and textile industries, recent years have seen a huge growth in its popularity in the food packaging sector. However, there, PLA is seen as a ‘temporary’ solution. Great for takeaway coffee cups and food containers at festivals, but not at all suited to longer-term food storage, due to its poor barrier properties (high permeability to water vapour and oxygen). To address this limitation, a cross-discipline team of researchers from Chile has developed a series of PLA nanocomposites that combine the biodegradable polymer with functionalised graphene oxide (GO). They report on their results in the July issue of Polymer Testing [DOI: 10.1016/j.polymertesting.2023.108066].
They started by producing graphene oxide which they then functionalized with two types of alkylamines (decylamine (DA) and octadecylamine (ODA)) and at two different temperatures; 25 °C (GO-DA1 and GO-ODA1) and 80 °C (GO-DA2 and GO-ODA2). These functionalised GO nanoparticle fillers were incorporated into PLA at different loadings (0.2, 0.7, 2 wt%) using the melt-mixing technique. The resulting composite films were prepared with a thickness of either 1.0 mm (for mechanical testing) or 0.1 mm (for permeability analysis), and underwent a series of other characterisation stages, including FTIR spectroscopy, thermogravimetric analysis and differential scanning calorimetry.
They found that temperature had a significant effect on the GO functionalisation stage – reactions carried out at 80 °C showed higher mass yields than at room temperature. Elemental analysis showed that functionalisation added nitrogen to the nanoparticles, reaching a maximum content (3.92 wt%) for GO-DA2. X-ray diffraction and FTIR supported this finding, confirming the presence of alkyl chains on the functionalised nanoparticles. Thermal analysis of the nanoparticles found that while GO exhibited significant mass loss (38 %) between 130 and 280 °C, functionalised GO was much more thermally stable – for example, mass loss reached just 12 % for GO-DA2 and 7 % for GO-ODA2. Calorimetry curves on the composite films showed that the degree of crystallinity of PLA-GO was lower than any of the PLA-functionalised GO films – this suggests that functionalisation promotes a higher affinity between the composite’s polymer and its fillers. Optical microscopy analysis of the composites confirmed this result.
The authors found that the mechanical properties of PLA composites decreased slightly as nanoparticle loading increased. They attribute this to four factors, “...the formation of agglomerates, the decrease in the crystallinity of the nanocomposites, the formation of cracks at the particle-polymer interface, and the plasticizing effect the alkyl chains present in the modified GO.” Barrier tests of the composites showed that the permeability to both oxygen and water vapour decreased significantly as nanoparticle loading increased. The composite PLA-GO-ODA2 exhibited the best overall barrier performance – at 0.7 wt%, the oxygen permeability decreased by 30.4 % compared to standard PLA. And using just 0.2 wt% of GO-ODA2 produced a 50.2 % reduction in water vapor permeability, compared to PLA alone.
The authors conclude, “This outstanding improvement in the barrier properties, beside other possible applications, make this material suitable for food packaging.”
---
Remilson Cruz, Muhammad Nisar, Humberto Palza, Mehrdad Yazdani-Pedram, Héctor Aguilar-Bolados, Raúl Quijada. “Development of bio degradable nanocomposites based on PLA and functionalized graphene oxide,” Polymer Testing 124 (2023) 108066. DOI: 10.1016/j.polymertesting.2023.108066