New biodegradable composites for food packaging

15 September 2014
Mara Cunha, José A. Covas, Loïc Hilliou, Marie-Alix Berthet, Ricardo Pereira, and António A. Vicente
Beer spent grain fibers are a biodegradable waste resource that forms breathable thin films by conventional extrusion film blowing under processing conditions scalable to industrial production.

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 is to be degraded 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.1Despite the relevance of this technology for the production of thin film for food packaging, there are no reports in the open literature about biocomposites designed for conventional extrusion film blowing. Indeed, existing biodegradable solutions rely on oil-based biodegradable plastics, or are not compostable.2 We transformed beer spent grains, a food industry by-product, into fibers,3 which we then used as fillers to reduce the cost and improve the viscoelastic properties of a commercial PHA.

We started by studying the processability of pre-mixes of PHA (ENMATTM Y1000P, TianAn Biologic Materials Company) with the fibers. We then selected two formulations and processed them in a laboratory blown film extrusion line (Periplast, Portugal).4 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.

We screened formulations for processability with a prototype mini-extrusion line with outputs in the range 80–300g/h,5comprising a volumetric screw feeder, a modular co-rotating twin screw extruder, a slit die, and a winding roll: see Figure 1(A). Note that a rheo-optical slit die was attached to it to allow in-line structural and viscoelastic characterization.6 We tested beer spent grain fiber (BSGF) contents between 1 and 20wt%. Optimized extrusion parameters included set temperature profile and draw down ratio (DDR), that is, the ratio between the haul off velocity and the melt velocity at the die exit. A temperature profile in excess of 180°C, which is the melting temperature of PHA, resulted in a melt with no strength, dripping out of the die: see Figure 1(B). However, targeting set temperatures between 170 and 180°C yielded good results. Composites formulated with more than 10wt% BSGF could not be processed. As shown by rheological measurements, more than 10wt% BSGF builds up a percolating network in the PHA matrix, converting the composite into a viscoelastic solid that cannot be extruded. The drawing ability of the composites was surprisingly good, and DDR as large as 50 could be achieved: see view from above in Figure 1(C), regardless of the BSGF content. Tensile testing of the extrudates showed that both Young's modulus and strain at break were not improved by increasing BSGF content to more than 5wt%. In view of these results, we selected the formulations containing 2 and 5wt% BSGF for film blowing tests.


(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.

During the extrusion-blown film studies, we varied both the take-up ratio (TUR)—the ratio between the take-up speed and the extrusion speed—and the blow-up ratio (BUR)—the ratio between the final bubble diameter and the diameter of the die lips, whereas the remaining parameters (set temperature profile, mass output, bubble cooling conditions) were kept constant. Relatively stable bubbles—see example in Figure 2(A)—could be obtained for a narrow range of low BUR and TUR: see Table 1. Examples of the films produced are presented in Figure 2(B). The mechanical properties measured both in the machine and transverse directions were similar and indicated that little molecular orientation was achieved. Young's modulus of the composite films was lower than that of the film made with PHA, whereas the strain at break was not significantly affected by the addition of BSGF: see Table 1. This drop in rigidity should be due to the poor adhesion between BSGF and the PHA matrix: see the scanning electron microscopy micrograph in Figure 2(C). Wide-angle x-ray diffraction showed the crystallinity of the composite films was reduced by adding the BSGF. 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.

Ranges of mechanical properties (Young's modulus, E, and strain at break, εB), measured in the machine direction, of water vapor permeability (WVP) and of oxygen permeability (O2) for films blown using the indicated ranges of the blow-up ratio (BUR) and take-up ratio (TUR).

BSGF content (wt%)BURTURE (GPa)εB(%)WVP (10−11g/(smPa))O2(10−12gm/(smPa)2)
02–38–125.5–6.70.8–1.01.2–1.70.5–0.6
21–36–72.7–4.40.9–1.21.3–41–2
52–34–52.9–3.90.7–0.82–51.5–6.5

In summary, we have performed a comprehensive study of 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, we are now working to produce multilayer films by co-extrusion, surface modification of the fibers being another possible route.


Authors

Mara Cunha
IPC/I3N Institute for Polymers and Composites University of Minho

José A. Covas
IPC/I3N Institute for Polymers and Composites University of Minho

Loïc Hilliou
IPC/I3N Institute for Polymers and Composites University of Minho

Loïc Hilliou is a researcher. His recent research focuses on the rheology of biodegradable and natural materials.

Marie-Alix Berthet
IATE Université Montpellier II

Ricardo Pereira
Centre of Biological Engineering University of Minho

António A. Vicente
Centre of Biological Engineering University of Minho


References

  1. M. Cunha, M.-A. Berthet, R. Pereira, J. A. Covas, A. A. Vicente and L. Hilliou, Development of polyhydroxyalkanoate/beer spent grain fibers composites for film blowing applications, Polym. Compos., 2014. First published online: 5 June. doi:10.1002/pc.23093

  2. N. Peelman, P. Ragaert, B. De Meulenaer, D. Adons, R. Peeters, L. Cardon, F. Van Impe and F. Devlieghere, Application of bioplastics for food packaging, Trends Food Sci. Tech. 32 (2), pp. 128-141, 2013.

  3. E. Pires, H. A. Ruiz, J. A. Teixeira and A. A. Vicente, A new approach on brewers' spent grains treatment and potential use as lignocellulosic yeast cells carriers, J. Agr. Food Chem. 60 (23), pp. 5994-5999, 2012.

  4. O. S. Carneiro, R. Reis and J. A. Covas, Small-scale production of a co-extruded biaxially oriented blown film, Polym. Test. 27 (4), pp. 527-537, 2008.

  5. R. M. Novais, J. A. Covas and M. C. Paiva, The effect of flow type and chemical functionalization on the dispersion of carbon nanofiber agglomerates in polypropylene, Composites A: Appl. Sci. Manuf. 43 (6), pp. 833-841, 2012.

  6. P. F. Teixeira, J. M. Maia, J. A. Covas and L. Hilliou, In-line particle size assessment of polymer suspensions during processing, Polym. Test. 37 (1), pp. 68-77, 2014.

DOI:  10.2417/spepro.005559