Abstract
To explore the potential of long chain polyhydroxyalkanoates as non-toxic food packaging materials, the characterization of polyesters prepared from a natural occurring polyhydroxylated C16 carboxylic acid (9,10,16-trihydroxyhexadecanoic or aleuritic acid) has been addressed. Such monomer has been selected to elucidate the reactivity of primary and secondary hydroxyl groups and their contribution to the structure and properties of the polyester. Resulting polyaleuritate films have been produced using an open mold in one-step, solvent-free self-polycondensation in melt state and directly in air to evaluate the effect of oxygen in their final physical and chemical properties. These polymers are amorphous, insoluble, and thermostable, being therefore suitable for solvent, and heat resistant barrier materials. Structurally, most of primary hydroxyls are involved in ester bonds, but there is some branching arising from the partial participation of secondary O-H groups. The oxidative cleavage of the vicinal diol moiety and a subsequent secondary esterification had a noticeable effect on the amorphization and stiffening of the polyester by branching and densification of the ester bond network. A derivation of such structural modification was the surface compaction and the reduction of permeability to water molecules. The addition of Ti(OiPr)4 as a catalyst had a moderate effect, likely because of a poor diffusion within the melt, but noticeably accelerated both the secondary esterification and the oxidative processes. Primary esterification was a high conversion bulk reaction while oxidation and secondary esterification was restricted to nearby regions of the air exposed side of cast films. The reason was a progressive hindering of oxygen diffusion as the reaction progresses and a self-regulation of the altered layer growth. Despite such a reduced extent, the oxidized layer noticeably increased the UV-vis light blockage capacity. In general, characterized physical properties suggest a high potential of these polyaleuritate polyesters as food preserving materials.
Highlights
The environmental and health threat caused by massive production, use, and disposal of fuel-based synthetic polymers is a major issue of concern in our society (Lithner et al, 2011)
The polyaleuritates prepared in this work were characterized by attenuated total reflection (ATR)-FTIR spectroscopy as polyhydroxylated fatty esters with characteristic ν(C=O) at 1,735 cm−1, ν(C-O-C) bands at 1,248 and 1,177 cm−1, and ν(C-O), and ν(O-H) of hydroxyls in the 1,050–1,110 cm−1 and 3,550–3,250 cm−1 regions, respectively, as well as intense ν(–CH2-) peaks around 2,930 and 2,854 cm−1 (Bellamy, 1975; Benítez et al, 2015c)
Compared to soluble thermoplastic short chain polyhydroxyalkanoates (PHAs) and poly(lactic acid) (PLA), polyaleuritates prepared in air are adequate materials to resist thermal sterilization procedures and the migration to foodstuff when used in food packaging
Summary
The environmental and health threat caused by massive production, use, and disposal of fuel-based synthetic polymers is a major issue of concern in our society (Lithner et al, 2011). Monomers conforming the biopolyester cutin are mostly C16 and C18 hydroxyacids, commonly midchain hydroxylated ω-hydroxyacids (Baker and Holloway, 1970; Walton and Kolattukudy, 1972), but there are other chemical groups such as unsaturations, epoxy, and secondary vicinal diols This later functionality is present in aleuritic (9,10,16trihydroxyhexadecanoic) acid, the main component of shellac, a natural resin with applications in food, pharmaceutics and coatings and a potential production of 50,000 tons/year Though natural occurring fatty hydroxyacids from plant cutin or shellac are still far from being a cost-effective feedstock for bulk packaging materials, the perspective may change when designing few microns-thick coatings in which the amount of material per container is reduced by 2–3 orders of magnitude In this scenario, aspects such as non-toxicity would make up for cost, as for instance, in the replacement of BPA internal varnishes in food cans. UV-visible curves were obtained by using a Cary 300 spectrometer equipped with a barium sulfate coated integrating sphere (Labsphere)
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