Physical-chemical and structural stability of poly(3HB-co-3HV) / (ligno-)cellulosic fibre-based biocomposites over successive dishwashing cycles

International audience

anglais. Plastic wastes are one of the causes of current environmental concerns, targeting in particular packagingapplications, including single-use packaging such as disposable dishes. In order to minimize the use ofresources, it is essential to lengthen the life cycle of packaging materials via several recycling and/or reuseclosed loops, before an ultimate composting and/or biodegradation step. Recent European Union decreesgo in this direction, e.g., with the article L. 541-10-17 of the Environmental code requiring a 20% reductiontarget for single-use plastic packaging set for producers, of which at least 50% must be obtained by reuse by31 December 2025.In this context, the goal of this work was to assess the impact of successive dishwashing cycles on thestructural and physical-chemical stability of PHBV/(ligno-)cellulosic fibre-based biocomposite materials. Theeffect of fibre composition, morphology, and content was investigated by selecting three types of commercial(ligno-)cellulosic fibres and two filler contents (20 and 40 wt%). For that purpose, PHBV-based films wereproduced by melt extrusion followed by a thermopressing step. In accordance with the standard NF EN12875-1, up to 125 dishwashing cycles were carried out. Films having undergone a certain number of cyclesin the dishwasher have been characterised under standardised testing conditions. The evolution of thecolour, the tensile properties, and the overall migration in two food liquid simulants, i.e., ethanol 50%(vol/vol) and ethanol 95% (vol/vol), was discussed in relation to the structural stability of materials, whichwas evaluated by changes in sample weight and thickness, thermal transition temperature, and crystallinityassessed by differential scanning calorimetry (DSC), as well as microstructures observed by scanning electronmicroscopy (SEM).It was shown that neat PHBV films were very stable, with negligible change in appearance and colour after125 dishwashing cycles. Regarding mechanical properties, only a slight decrease of rigidity and ductilityduring the first 5 cycles was noticed, then remaining stable up to 125 dishwashing cycles. Considering OMvalues, which is the key point to ensure food safety, they remained lower than the overall migration limit of10 mg.dm−2 established by the UE Committee for the two tested food liquid simulants, up to 50 dishwashercycles. Anyway, the present work highlights that biodegradability and compostability are not incompatiblewith recycling and reuse processes. The incorporation of increasing amounts of (ligno-)cellulosic fillersnegatively impacts the overall stability of PHBV-based materials. Over the 10 first successive dishwashingcycles, biocomposite films became rapidly and obviously less transparent and very brittle, with increasedsurface rouhghness. The loss in biocomposite performance was ascribed to the degradation of the polymermatrix through hydrolysis reactions, as well as by the swelling of hydrophilic (ligno-)cellulosic fillers, bothinduced by the sorption of water. High-purity cellulose fibres, with high crystallinity and aspect ratio, shouldbe preferred to limit water sorption and the release of water-soluble components during dishwashing. In thebest case, up to only 10 cycles could be applied. Worth to note that to be able to conclude on the possibilityto reuse PHBV-based materials as food contact materials, several additional investigations should be carriedout. Once the reusability can be fully demonstrated, collection and sorting steps should be established.