Release of carbon nanoparticles of different size and shape from nanocomposite poly(lactic) acid film into food simulants

ABSTRACT

Poly(lactic) acid (PLA) film with 2 wt% mixed carbon nanofillers of graphene nanoplates (GNPs) and multiwall carbon nanotubes (MWCNTs) in a weight ratio of 1:1 with impurities of fullerene and carbon black (CB) was produced by layer-to-layer deposition and hot pressing. The release of carbon nanoparticles from the film was studied at varying time–temperature conditions and simulants. Migrants in simulant solvents were examined with laser diffraction analysis and transmission electron microscopy (TEM). Film integrity and the presence of migrants on the film surfaces were visualised by scanning electron microscopy (SEM). The partial dissolution of PLA polymer in the solvents was confirmed by swelling tests and differential scanning calorimetry (DSC). Nanoparticle migrants were not detected in the simulants (at the LOD 0.020 μm of the laser diffraction analysis) after migration testing at 40°C for 10 days. However, high-temperature migration testing at 90°C for 4 h provoked a release of GNPs from the film into ethanol, acetic acid and oil-based food simulants. Short carbon nanotubes were observed rarely to release in the most aggressive acetic acid solvent. Obviously, the enhanced molecular mobility at temperatures above the glass transition and partial dissolution of PLA polymer by the food simulant facilitate the diffusion processes. Moreover, shape, size and concentration of nanoparticles play a significant role. Flexible naked GNPs (lateral size 100–1000 nm) easily migrate when the polymer molecules exhibit enhanced mobility, while fibrous MWCNTs (> 1 μm length) formed entangled networks on the film surfaces as the PLA polymer is partly dissolved, preventing their release into food simulants. The impurities of fullerenes and CB (5–30 nm) were of minor concentration in the polymer, therefore their migration is low or undetectable. The total amount of released migrants is below overall migration limits.

Graphical Abstract

KEYWORDS:

• Migration
• PLA
• graphene nanoplates
• multiwall carbon nanotubes
• food simulants
• swelling
• dissolution
• laser diffraction
• TEM
• SEM
• DSC

Acknowledgements

This research was supported by the H2020-696656 Graphene Core 1 and the H2020MSCA-RISE-734164 Graphene 3D projects. The authors acknowledge support given by the Bulgarian Science Fund co-funding the COST CA15114 (AMICI) for the safety study and FP7-316987 AComIn for laser diffraction analysis.

Disclosure statement

No potential conflict of interest was reported by the authors.