From Science & Research
From Science & Research PLA 3051D PLA - 40% AII- 3% B104 Residual specimens Figure 3 (A-C): UL94 HB fire testing: specimens (~3.1 mm thickness) of (a) neat PLA burning with dripping and without char formation; (B) PLA- 40% CaSO4 AII (9 μm) - 3% B104 (nano)composites burning without any dripping and with intensive charring (as shown on the residue remaining at the end of the test (C)) HB classification (max. admissible value of 40 mm/min), together with the total absence of dripping and the formation of an intensive char (Figure 3). On one hand, the specimen samples based on either unfilled PLA or PLA filled only with AII (even at content as high as 40-50 wt%) burned with intensive dripping (continuous formation of burning droplets) and without charring. On the other hand, even if no flamed droplet was generated upon burning the binary PLA-OMLS nanocomposites, their burning rate increased preventing HB classification [5, 6]. Therefore, only the ternary PLA-AII-OMLS (nano)composites reached HB classification and displayed intensive charring attesting for the unique synergistic effect between the CaSO 4 microfiller and organo-modified nanoclay. In relation to other key-properties, it is firmly believed that these novel PLA-based (nano)composites are perfectly suited for technical applications (e.g., electronic devices, electrical accessories, automotive parts, household appliances, etc.) due to their thermal stability and excellent processing ability evidenced using traditional techniques such as extrusion, injection and compression molding. A B C Case study 2: PLA-ZnO nanocomposite films and fibers: anti-UV and antibacterial properties ZnO nanoparticles are well-known environmentally friendly and multifunctional inorganic additives that could be considered as nanofillers for PLA providing properties like antibacterial action or intensive ultraviolet absorption. However, ZnO as well as other Zn derivatives are known as very efficient catalysts in ring-opening polymerization of lactide but also in ‘unzipping’ depolymerization of PLA. Indeed, preliminary studies revealed that addition of untreated ZnO nanoparticles into PLA at melt-processing temperature led to severe degradation of the polyester matrix, i.e., drastic reduction of PLA molecular weight, resulting in a sharp reduction of their thermo-mechanical characteristics [7]. Noteworthy, to make PLA matrix less susceptible to the catalytic action of ZnO during the melt blending process and any subsequent film/fiber processing, various filler surface treatments with selected additives (stearic acid, stearates, (fatty) amides, etc.) were tested with relatively low effectiveness. Remarkably, ZnO surface-treated by triethoxy caprylylsilane (i.e., commercial grade Zano 20 Plus supplied by Umicore Zinc Chemicals) leads to PLA-based nanocomposites characterized by very good preservation of the intrinsic molecular parameters of PLA and related physicochemical characteristic features. Furthermore, the surface-coated ZnO nanoparticles proved to finely and regularly disperse within the polyester matrix as highlighted by TEM (Figure 4). Additionally, whatever the nature of the PLA matrix, i.e., spinning or extrusion grade, the nanocomposites filled from 1 to 3 % surface-treated ZnO show mechanical properties, e.g., a tensile strength in the range 55 - 65 MPa, at least comparable and even somewhat higher than those obtained for the neat polyester matrix [7]. Noticeable, these nanocomposites show the onset of thermal degradation (T 5% ) at significantly higher temperature (from 20 to 40 °C) with respect to the samples containing untreated ZnO. Such improvements represent a real interest in the perspective of their utilization in production of films or fibers, and are mainly attributed to the effect of the –Si-O-Si-O- layers that cover the nanofiller surface and behave as a protecting barrier limiting the catalytic effect of ZnO able to promote unzipping of the nearby PLA chains. Interestingly, the related PLA-ZnO nanocomposite films as produced by compression molding or extrusion, proved to be characterized by very effective anti-UV action (Figure 5), in fact a total anti-UV protection is obtained for an amount of nanofiller as low as 1%. On another hand, PLA-ZnO nanocomposites have been also melt-spun and a highly efficient antibacterial protection on knitted fabrics was evidenced to both gram positive and gram negative bacteria [7]. 48 bioplastics MAGAZINE [01/12] Vol. 7
From Science & Research Further prospects: PLA-based hybrid nanocomposites Other nano-reinforcements for PLA are under development, but the most extensively studied so far, remain natural clays (like montmorillonite, sepiolite and halloysite) or carbon-based nanoparticles, mostly carbon nanotubes (CNT) and expanded/ exfoliated graphite. As illustration, exfoliated graphite as nanofillers combine the lower price and the layered structure of clay nanoplatelets with the superior thermal and electrical performances of CNT, whereas other specific end-use properties, e.g., mechanical rigidity, lower coefficient of friction, better abrasion resistance, have been highlighted. Also, PLA-expanded graphite (EG) nanocomposites proved to be characterized by increased kinetics of crystallization as well as thermo-mechanical properties allowing the application of these materials at higher temperature [8]. Furthermore, co-addition of EG and CNT into PLA paves the way to hybrid nanocomposites characterized by an interesting set of properties: higher tensile strength and rigidity, improved FR, conductive electrical characteristics even in presence of tiny amount of CNT. Again, the extent of the nanoparticle dispersion throughout the matrix remains a challenge where adequate surface treatment and/or addition of interfacial compatibilizers represent the best tools to get rid of filler aggregation. Conclusion Following the recent expansion of bioplastics and in response to the demand for enlarging PLA applications, it has been emphasized that PLA can be effectively melt-blended with selected micro- and nano-fillers to produce novel bio(nano) composites. Successful up-scaling of laboratory results via continuous twin-screw extrusion technology has been achieved paving the way to industrial applications. In this contribution, two case studies are discussed: i) PLA filled with CaSO 4 (AII) and selected organo-modified clays yielding high performance (nano) composites, and ii) PLA-(surface-treated) ZnO nanocomposites leading to nanocomposite films and fibers with specific end-use properties : anti-UV protection and antibacterial action. Based on these illustrations, very promising developments in the synergy aspects are clearly expected from the combination of nanofillers and more efforts are to be consented in this direction. 90 80 0% Zn0 neat PLA 70 1% Zn0 60 3% Zn0 50 40 30 on PLA films (0.2 - 0.3 mm thickness) 20 10 PLA - ZnO Wavelength (nm) 0 200 300 400 500 600 700 800 Transmittance (%) Figure 5: UV-vis spectra of selected samples of PLA-ZnO (silane treated) films compared to neat PLA evidencing total anti-UV protection Figure 4: TEM picture of PLA (spinning grade) -1% ZnO (silane treated) attesting for good nanofiller dispersion into PLA matrix http://morris.umons.ac.be/CIRMAP www.materianova.be Authors thank the Wallonia Region, Nord-Pas de Calais Region and European Community for the financial support in the frame of the INTERREG – MABIOLAC and NANOLAC projects. They thank all partners, especially to ENSC Lille and ENSAIT- Roubaix (France), for technical/ scientific support and helpful discussions, and all mentioned companies for supplying raw materials. CIRMAP acknowledges supports by the Région Wallonne in the frame of OPTI²MAT program of excellence, by the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 6/27) and by FNRS- FRFC. References 1. Platt D. Biodegradable Polymers - Market report. Smithers Rapra Limited UK, Shawbury, Shrewsbury, Shropshire, 2006. 2. Madhavan Nampoothiri K, Nair NR, John RP. Biores. Tech. 2010;101:8493–501. 3. Dubois Ph, Murariu M. JEC Composites Magazine 2008;45:66-9. 4. Murariu M, Da Silva Ferreira A, Degée Ph, Alexandre M, Dubois Ph. Polymer 2007;48(9):2613-8. 5. Murariu M, Bonnaud L, Yoann P, Fontaine G, Bourbigot S, Dubois Ph. Polym. Degra.d Stabil. 2010;95:374-81. 6. Dubois Ph, Murariu M, Alexandre M, Degée Ph, Bourbigot S, Delobel R, Fontaine G, Devaux E. Polylactide-based compositions. WO Patent 095874 Al, 2008. 7. Murariu M, Doumbia A, Bonnaud L, Dechief AL, Paint Y, Ferreira M, Campagne C, Devaux E, Dubois Ph. Biomacromolecules 2011;12:1762-71. 8. Murariu M, Dechief AL, Bonnaud L, Paint Y, Gallos A, Fontaine G, Bourbigot S, Dubois Ph. Polym. Degrad. Stabil. 2010;95:889-900. bioplastics MAGAZINE [01/12] Vol. 7 49
