Mike Tolinski in his Journal of Renewables Materials recently reported that his lab transformed beer spent grains, a food industry by-product, into fibers, which were then used as fillers to reduce the cost and improve the viscoelastic properties of a commercial PHA.
Polyhydroxyalkanoates (PHAs) are biopolyesters produced by micro-organisms cultivated in carbon substrates. PHAs degrade in soil, sludge, or seawater at room temperature.
As such, they are good candidates for food packaging applications where the container would ideally ends its life in the same stream as the food product. However, PHAs are expensive and possess reduced melt processability.
Thus, most PHA-based biocomposites are formulated with cheap fillers and other additives and then processed by compression or injection molding.Despite the relevance of this technology for the production of thin film for food packaging, there is currently no example in the public domain of biobased compostable resin designed for conventional extrusion film blowing. Indeed, existing biodegradable solutions rely on oil-based biodegradable plastics, or are not compostable.
The Beer Spent Grains Fibers (BSGF) and PHA were compounded into a number of blend formulations.
Two formulations were selected and processed in a laboratory blown film extrusion line (Periplast, Portugal). Films obtained under different operating conditions were produced and analyzed in terms of structural, mechanical, and barrier properties to assess their suitability for food packaging.
Below are reproduced some of the most interessant findings of the study.
(A) Mini-extrusion line used to assess the processability of polyhydroxyalkanoate/beer spent grain fiber (PHA/BSGF) composites. (B) Photograph showing the composite has no melt strength due to an excessively hot temperature profile. (C) Photograph from above, showing the composite has good drawability.
Incorporating BSGF increased the permeability of the PHA films to oxygen and water vapor. For example, the oxygen permeability increased tenfold for the film containing 5wt% BSGF. These results indicate that the film gas barrier properties can be tailored by modifying the BSGF content.
The range of attainable permeability values is suitable for respiring film applications, such as packaging for fresh fruits and vegetables.
(A) Bubble blown from a composite containing 2wt% of BSGF. (B) Samples of films produced with extrusion film blowing, from left to right: PHA, PHA with 2wt% BSGF, and PHA with 5wt% BSGF. (C) Scanning electron microscopy image of a film blown from a PHA composite containing 5wt% BSGF.
In summary, this comprehensive study has proven the processability of bio-sourced and compostable PHA/BSGF composites, producing composite films containing 5wt% BSGF using conventional extrusion film blowing, under conditions scalable to industrial production, and with promising properties for fruit and vegetable packaging.
To improve the films’ mechanical and barrier properties, the lab team is now working to produce multilayer films by co-extrusion, surface modification of the fibers being another possible route.
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