Abstract
In the present study, three different newly developed copolymers of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with 20, 40, and 60 mol % contents in 3-hydroxyvalerate (3HV) were produced by the biotechnological process of mixed microbial cultures (MMCs) using cheese whey (CW), a by-product from the dairy industry, as feedstock. The CW-derived PHBV copolyesters were first purified and then processed by solution electrospinning, yielding fibers of approximately 2 μm in cross-section in all cases. The resultant electrospun PHBV mats were, thereafter, post-processed by annealing at different temperatures, below their maximum of melting, selected according to their 3HV content in order to obtain continuous films based on coalesced fibers, so-called biopapers. The resultant PHBV films were characterized in terms of their morphology, crystallinity, and mechanical and barrier properties to assess their potential application in food packaging. The CW-derived PHBV biopapers showed high contact transparency but a slightly yellow color. The fibers of the 20 mol % 3HV copolymer were seen to contain mostly poly(3-hydroxybutyrate) (PHB) crystals, the fibers of the 40 mol % 3HV copolymer a mixture of PHB and poly(3-hydroxyvalerate) (PHV) crystals and lowest crystallinity, and the fibers of the 60 mol % 3HV sample were mostly made of PHV crystals. To understand the interfiber coalesce process undergone by the materials during annealing, the crystalline morphology was also assessed by variable-temperature both combined small-angle and wide-angle X-ray scattering synchrotron and Fourier transform infrared experiments. From these experiments and, different from previously reported biopapers with lower 3HV contents, all samples were inferred to have a surface energy reduction mechanism for interfiber coalescence during annealing, which is thought to be activated by a temperature-induced decrease in molecular order. Due to their reduced crystallinity and molecular order, the CW-derived PHBV biopapers, especially the 40 mol % 3HV sample, were found to be more ductile and tougher. In terms of barrier properties, the three copolymers performed similarly to water and limonene, but to oxygen, the 40 mol % sample showed the highest relative permeability. Overall, the materials developed, which are compatible with the Circular Bioeconomy organic recycling strategy, can have an excellent potential as barrier interlayers or coatings of application interest in food packaging.
Highlights
Nowadays, the use of alternative materials to conventional plastics is increasingly important due to the environmental issues associated to the extensive use of single-use plastics
In the Fourier transform infrared (FTIR) spectra of the PHBV copolyesters taken at room temperature, shown in Figure 6a, the strongest peak observed at nearly 1720 cm−1 is assigned to the conformationally sensitive stretching vibration of the carbonyl group (C O).[73]
Three PHBV copolyesters with different 3HV contents, that is, 20, 40, and 60 mol %, were successfully produced at a pilot plant scale using the technology of mixed microbial cultures (MMCs) fed with cheese whey (CW), a by-product of the dairy industry
Summary
The use of alternative materials to conventional plastics is increasingly important due to the environmental issues associated to the extensive use of single-use plastics. This process is based on the application of electrostatic forces to polymer solutions through the action of a high-voltage electric field,[35] where the fibers formation is affected by both the solution properties and process conditions.[36] The high surface-to-volume ratios of the fibers, the controllable pore sizes, and the possibility to nanoencapsulate different substances make electrospinning very promising for the formation of active and bioactive materials with improved performance.[37] More recently, it has been described that the electrospun fiber mats can be transformed into continuous films by the application of a thermal posttreatment below the biopolymer’s Tm, referred as annealing.[38] Since the fiber-based morphology is preserved in the resultant electrospun film, the annealed mats of naturally derived polymers are called “biopapers”.38 These have the advantage of being made purely of non-cellulosic biofibers that do not undergo aggressive chemical treatments, as is the case of the traditional paper.[39] biopapers show better optical and barrier properties and higher ductility and toughness than traditional paper packaging and similar barrier performance compared to films of same materials obtained by conventional melt processing or solvent casting.[40] The generation of new MMC-derived PHAs, with targeted increased HV contents, is known to yield more ductile materials, which, for the time being, still contain a number of cellular impurities. From a technological view point, the objective was to offer property balanced, more sustainable, and cost affordable options to commercial PHAs films processed by conventional melt compounding strategies and to traditional papers
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