Abstract
Sustainable coatings for metal food packaging were prepared from ZnO nanoparticles (obtained by the thermal decomposition of zinc acetate) and a naturally occurring polyhydroxylated fatty acid named aleuritic (or 9,10,16-trihydroxyhexadecanoic) acid. Both components reacted, originating under specific conditions zinc polyaleuritate ionomers. The polymerization of aleuritic acid into polyaleuritate by a solvent-free, melt polycondensation reaction was investigated at different times (15, 30, 45, and 60 min), temperatures (140, 160, 180, and 200 °C), and proportions of zinc oxide and aleuritic acid (0:100, 5:95, 10:90, and 50:50, w/w). Kinetic rate constants calculated by infrared spectroscopy decreased with the amount of Zn due to the consumption of reactive carboxyl groups, while the activation energy of the polymerization decreased as a consequence of the catalyst effect of the metal. The adhesion and hardness of coatings were determined from scratch tests, obtaining values similar to robust polymers with high adherence. Water contact angles were typical of hydrophobic materials with values ≥94°. Both mechanical properties and wettability were better than those of bisphenol A (BPA)-based resins and most likely are related to the low migration values determined using a hydrophilic food simulant. The presence of zinc provided a certain degree of antibacterial properties. The performance of the coatings against corrosion was studied by electrochemical impedance spectroscopy at different immersion times in an aqueous solution of NaCl. Considering the features of these biobased lacquers, they can be potential materials for bisphenol A-free metal packaging.
Highlights
Metal packaging represents globally ∼15% of the packaging, being an important business that is estimated to reach ∼58 billion US dollars in 2024.1 Leading manufacturers of metal cans are North America and Europe, with market shares of 32 and 30%, respectively, of a global production of ∼430 billion cans in 2020.1 Metal packaging forms a barrier against light, oxygen, and pathogens that protects canned food from the environment and increases its shelf-life, keeping food quality for long times
We report the fabrication process of green, zinc polyaleuritate ionomer-based coatings for metal packaging of foodstuffs as potential substitutes for bisphenol A (BPA) lacquers
The reflection peaks due to the formation of Zinc oxide (ZnO) NPs arose from the background after the annealing, revealing that the prepared NPs are characterized by a hexagonal crystalline structure, typical of a zincite phase
Summary
Metal packaging represents globally ∼15% of the packaging, being an important business that is estimated to reach ∼58 billion US dollars in 2024.1 Leading manufacturers of metal cans are North America and Europe, with market shares of 32 and 30%, respectively, of a global production of ∼430 billion cans in 2020.1 Metal packaging forms a barrier against light, oxygen, and pathogens that protects canned food from the environment and increases its shelf-life, keeping food quality for long times. The increase of adhesion induced by Zn has been described, which is commercially exploited in analogous structures such as zinc polycarboxylates (mainly polyacrylates), usually known as dental luting cement.[55] the adhesion of these zinc polycarboxylates to aluminum-based surfaces has been reported as very high.[56] Scratch hardness tests (Figures 5C and S3) showed similar results: the AZ-100 sample exhibited the worst performance with a value of ∼91 MPa, while AZ-95 and AZ-90 had similar behavior with values of ∼306 and ∼327 MPa, respectively (i.e., an increase of ∼236 and ∼259%, respectively) Such reinforcement of mechanical properties produced by different zinc carboxylates has been previously reported for several rubbers, elastomers, and vulcanizate thermoplastics.[57−60] the AZ values of scratch hardness were compared to those of other manmade polymers and resins: polyethylene (PE), polybutene (PB), two different types of bisphenol A-based epoxy resins (Duroglass P5/1 labeled as Epoxy-1 and Ampreg 26 labeled as Epoxy-2), polycarbonate (PC), poly(methyl methacrylate) (PMMA), styrene-acrylonitrile copolymer (SAN), and melamine formaldehyde (MF).[61] As observed, AZ-100 shows similar values to soft polymers such as PB and Epoxy-1, while AZ-90 and AZ-95 are comparable to very robust SAN and MF, overcoming the performances of typical epoxy resins. There are no reports about aleuritic acid toxicity when used as a food additive, the maximum dose for shellac (mainly composed of aleuritate units) to avoid toxic effects is 5000 ppm.[75]
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