Industrial-scale preparation of nanocellulose-reinforced biocomposites

22 August 2017
Pejman Heidarian, Tayebeh Behzad, Keikhosro Karimi, and Mohini Sain
A novel method is used to fabricate recycled polylactic acid/cellulose nanofiber composites and to improve the overall properties of the biofilms.

Polylactic acid (PLA)—an aliphatic polyester that can be obtained from renewable resources—has recently emerged as an environmentally friendly alternative for most petroleum-based polymers. Despite the many advantages of PLA, its relatively poor mechanical properties mean that its use is limited in some applications.1, 2 To address this problem, nanocellulose has gained considerable attention over the last few years as a suitable bioreinforcement for the manufacture of high-performance PLA (and other) biocomposites.3

Nanocellulose, which exists naturally in plant cell walls, can be isolated via various chemomechanical extraction methods.4 Depending on the precise nature of the treatment, a range of nanocellulose morphologies (displaying different mechanical properties) can be obtained, such as cellulose nanowhiskers and cellulose nanofibers (CNFs).5, 6 Several previous studies have focused on changing the properties of PLA by the addition of nanocellulose, via a number of methods. For example, the use of melt compounding,7, 8 solvent casting,9,10 and kneading and calendaring7, 11,12 techniques have been studied. Nevertheless, the search for a reliable industrial-scale method that can be used to achieve a good dispersion of nanocellulose in a PLA matrix is still ongoing. Moreover, the large amount of chemicals consumed during the preparation of nanocellulose-reinforced PLA samples is a major drawback of current techniques.

In this work,13 we have therefore developed a new industrial-scale procedure for the preparation of a recycled PLA (rPLA) biocomposite film. Although rPLA is an extremely environmentally friendly material, the recycling process it undergoes means that mechanical weaknesses are expected to emerge during its usage. To ameliorate these shortcomings, we have examined the influence of including bagasse CNFs as novel reinforcements in the rPLA material. We used an optimized procedure to isolate the nanocellulose from bagasse,6 and then used the solution-casting technique (with an acetone aqueous solution) to prepare CNF/rPLA masterbatches. From microscope observations of our samples, we find that the CNFs have web-like structures and an average diameter of about 35nm. This means that each isolated bagasse CNF chain consists of eight or nine bundles of elementary cellulose chains.14 In addition, since the cellulose backbone is entirely covered by hydroxyl groups, we believe that this morphology can provide very active surfaces for interactions with the rPLA chains.

We then used a scanning electron microscope to examine the fracture surfaces of the prepared samples (see Figure 1). Our images reveal that the fracture surface of the neat rPLA biofilm—see Figure 1(a)—was completely smooth. In addition, we find that the roughness of the surfaces was substantially increased upon addition of the CNFs: see Figure 1(b) to (d). This confirmed the interfacial interactions between the bagasse CNFs and the rPLA. Furthermore, our results indicate that the nanocomposite samples containing 1 and 3wt% CNFs—as shown in Figure 1(b) and (c)—exhibited morphologies that are much better homogenized than the 5wt% CNF sample shown in Figure 1(d). We believe that the poor homogenization of the 5wt% sample is caused by the low distribution/aggregation ratio of the CNFs in the rPLA, which created numerous voids in the fracture surface of the sample.13


Scanning electron microscope images of the nanocomposite samples. Images are shown for (a) the neat recycled polylactic (rPLA) sample, and for rPLA/cellulose nanofiber (CNF) composites containing (b) 1wt%, (c) 3wt%, and (d) 5wt% CNFs.

The results of the mechanical tests we conducted on our rPLA/CNF samples are illustrated in Figure 2. The typical stress–strain curves we obtained demonstrate that our use of bagasse CNFs as natural reinforcements improved the strength and modulus of the rPLA, even at low fiber concentrations (i.e., 1wt%). We attribute this result to the high crystallinity and surface area of CNFs.8, 15 Specifically, we measure—see Figure 2(a) and (b)—the maximum strength (32.6MPa) and modulus (716.5MPa) values for the 3wt% CNF composite. These values correspond to an increase of 36.4 (strength) and 35.8% (modulus) from the values for the neat rPLA sample. In contrast, the elongation at break and work of fracture of the CNF-containing nanocomposites are about 35.5 and 33% less (respectively) than that of the neat PLA sample: see Figure 2(c) and (d). In other words, addition of the bagasse CNFs caused the nanocomposites to become more brittle. This is caused by the presence of the rigid nanocrystalline fibers within the flexible matrix, which leads to incremental states at which microcrystalline structures can grow in the rPLA. From photographs of our samples, we also clearly see that the opacity of the samples increases with increasing CNF content.


(a) Tensile strength, (b) Young's modulus, (c) elongation at break, and (d) work of fracture results for the neat rPLA (i.e., 0wt% CNF) sample, as well as the 1, 3, and 5wt% rPLA/CNF nanocomposites.

We also performed a dynamic mechanical analysis to study the dynamical properties, including the storage modulus and tan delta (i.e., damping), of our nanocomposites (see Figure 3). Based on our results—see Figure 3(a)—we find that the storage modulus decreased drastically at the glass transition temperature of all the samples (i.e., at 59.5, 61.9, and 64°C for the 0, 1, and 3wt% CNF rPLA biofilms, respectively). This transition is caused by the segmental immobility of the polymer chains, which arises because of the increasingly rigid state that occurs between the rPLA matrix and the CNFs.8 We see another conspicuous change in the storage modulus of the samples at about 100°C, which is caused by the cold crystallization of amorphous chains in the rPLA.12, 15 Similarly, the measured tan delta peak of the samples—see Figure 3(b)—is shifted to slightly higher temperatures upon addition of the CNFs. We attribute this result mainly to chain entanglement between the CNFs and rPLA.15


Dynamic mechanical analysis for the rPLA and rPLA/CNF 1, 3, and 5wt% nancomposite samples. Measured (a) storage modulus (log scale) and (b) tan delta (damping) results are shown.

In the final part of our work, we investigated the biodegradability of our nanocomposites. We followed the methodology of a previous PLA biodegradation study,15 in which a high rate of hydrolytic degradation was achieved with soil at a temperature of 58±2°C and 60% humidity. Indeed, the main driving force for PLA hydrolytic degradation is its high water-uptake capability, which causes ester cleavage and the formation of small oligomers.1, 15 Photographs of our rPLA and rPLA/CNF samples—after different periods of biodegradation in the soil—are shown in Figure 4. We see the onset of hydrolytic degradation and brittleness (consequences of water diffusing from the soil) in the samples after 30 days. We also measure a decreased weight-loss percentage for the rPLA and the nanocomposites after 60 days, with the biggest loss for the neat PLA sample. The lower biodegradation rate of the nanocomposites is caused by their greater diffusion resistance compared with the unreinforced sample. The greatest level of biodegradation we observed occurred in the rPLA sample after 75 days. We suggest that the water-absorption process begins in the amorphous domains of the polymer, and the presence of nanocellulose between these domains in the nanocomposite samples therefore delays their biodegradation (in keeping with previously presented results15).16


Photographs of the rPLA and rPLA/CNF 1, 3, and 5wt% nancomposite samples at different stages of biodegradation. The samples are shown once they had been recovered from soil, after 14, 30, 60, or 75 days (samples not subjected to biodegradation are shown in the top row for comparison).

In summary, we have successfully used an industrial-scale approach to prepare CNF-reinforced rPLA biocomposites. Our morphological results reveal that the nanofibers were appropriately dispersed within the rPLA matrix, up to a concentration of 3wt%. In addition, we demonstrated that the addition of CNFs (even at very low concentrations) has a positive influence on the mechanical and thermo-mechanical properties of the rPLA biofilms. Moreover, we find that the hydrolytic degradation of rPLA is decreased by the addition of CNFs. Our production method for CNF-reinforced rPLA biocomposites is therefore a promising approach for the preparation of high-performance biofilms for use in various industries. To that end, we are now planning to investigate the potential of our biofilms for film packaging applications.


Authors

Pejman Heidarian
Department of Chemical Engineering, Isfahan University of Technology

Tayebeh Behzad
Department of Chemical Engineering, Isfahan University of Technology

Keikhosro Karimi
Department of Chemical Engineering, Isfahan University of Technology

Mohini Sain
Department of Chemical Engineering, University of Toronto


References

  1. J.-M. Raquez, Y. Habibi, M. Murariu and P. Dubois, Polylactide (PLA)-based nanocomposites, Prog. Polym. Sci. 38, pp. 1504-1542, 2013.

  2. R. Liu, Y. Peng and J. Cao, Thermal stability of organo-montmorillonite-modified wood flour/poly(lactic acid) composites, Polym. Compos. 37, pp. 1971-1977, 2016.

  3. M. A. Hubbe, O. J. Rojas, L. A. Lucia and M. Sain, Cellulosic nanocomposites: a review, Bioresources 3, pp. 929-980, 2008.

  4. N. Johar, I. Ahmad and A. Dufresne, Extraction, preparation and characterization of cellulose fibres and nanocrystals from rice husk, Indust. Crops Prod. 37, pp. 93-99, 2012.

  5. M. Jonoobi, K. Oksman Niska, J. Harun and M. Misra, Chemical composition, crystallinity, and thermal degradation of bleached and unbleached kenaf bast (Hibiscus cannabinus) pulp and nanofibers, Bioresources 4, pp. 626-639, 2009.

  6. P. Heidarian, T. Behzad and K. Karimi, Isolation and characterization of bagasse cellulose nanofibrils by optimized sulfur-free chemical delignification, Wood Sci. Technol. 50, pp. 1071-1088, 2016.

  7. A. Iwatake, M. Nogi and H. Yano, Cellulose nanofiber-reinforced polylactic acid, Compos. Sci. Technol. 68, pp. 2103-2106, 2008.

  8. M. Jonoobi, J. Harun, A. P. Mathew and K. Oksman, Mechanical properties of cellulose nanofiber (CNF) reinforced polylactic acid (PLA) prepared by twin screw extrusion, Compos. Sci. Technol. 70, pp. 1742-1747, 2010.

  9. D. Y. Liu, X. W. Yuan, D. Bhattacharyya and A. J. Easteal, Characterisation of solution cast cellulose nanofibre-reinforced poly(lactic acid), Express Polym. Lett. 4, pp. 26-31, 2010.

  10. P. Qu, Y. Gao, G.-F. Wu and L.-P. Zhang, Nanocomposites of poly(lactic acid) reinforced with cellulose nanofibrils, Bioresources 5, pp. 1811-1823, 2010.

  11. K. Okubo, T. Fujii and E. T. Thostenson, Multi-scale hybrid biocomposite: processing and mechanical characterization of bamboo fiber reinforced PLA with microfibrillated cellulose, Compos. Part A: Appl. Sci. Manuf. 40, pp. 469-475, 2009.

  12. L. Suryanegara, A. N. Nakagaito and H. Yano, The effect of crystallization of PLA on the thermal and mechanical properties of microfibrillated cellulose-reinforced PLA composites, Compos. Sci. Technol. 69, pp. 1187-1192, 2009.

  13. P. Heidarian, T. Behzad, K. Karimi and M. Sain, Properties investigation of recycled polylactic acid reinforced by cellulose nanofibrils isolated from bagasse, Polym. Compos., 2017.

  14. A. Dufresne, Nanocellulose: a new ageless bionanomaterial, Mater. Today 16, pp. 220-227, 2013.

  15. A. P. Mathew, K. Oksman and M. Sain, Mechanical properties of biodegradable composites from poly lactic acid (PLA) and microcrystalline cellulose (MCC), J. Appl. Polym. Sci. 97, pp. 2014-2025, 2005.

  16. K. M. Z. Hossain, I. Ahmed, A. J. Parsons, C. A. Scotchford, G. S. Walker, W. Thielemans and C. D. Rudd, Physico-chemical and mechanical properties of nanocomposites prepared using cellulose nanowhiskers and poly(lactic acid), J. Mater. Sci. 47, pp. 2675-2686, 2012.

DOI:  10.2417/spepro.006960