PLA nanocomposites

From Science & Research Figure 1: Principal steps in realization of PLA-gypsum AII-clay (nano)composites via melt-compounding technology in a co-rotating twin-screw extruder

(1) Gypsum AII + clays (dry-mixing) Drying all components

(2) Gravimetric dosing PLA and AII - clay

(3) Melt compounding in twin-screw extruder Leistritz type ZSE 18 HP-40D (ø=18 mm, L/D=40)

(4) Granulating (granules for injection molding)

PLA nanocomposites Tailored with specific end-use properties by Philippe Dubois, Marius Murariu Laboratory of Polymeric and Composite Materials Center of Innovation and Research in Materials and Polymers (CIRMAP) University of Mons (UMONS) & Materia Nova Research Center Mons, Belgium

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bioplastics MAGAZINE [01/12] Vol. 7

The ‘green’ challenge: polylactide (PLA)-based (nano)composites Polylactide or polylactic acid (PLA) is currently receiving considerable attention for rather conventional utilizations such as packaging materials as well as production of textile fibers, and more recently PLA has attracted increased interest for technical applications as well. [1-3] Actually, novel grades of PLA and related high performance PLA-based materials with higher added value are continuously searched for engineering applications such as electronic devices, electrical accessories, automotive parts, household appliances, etc. Consequently, the profile of PLA properties need to be tuned up for specifically reaching the end-user demands, and the combination of PLA with micro- and/or nano-fillers together with either flame retardants, impact modifiers, plasticizers or even other (bio)polymers represents a straightforward and readily scalable technical approach [2-8]. It is worth noting that the University of Mons (UMONS), through both the Center of Innovation and Research in Materials and Polymers (CIRMAP) and Materia Nova center, has significantly contributed to the field of bio(nano) composites. This involvement is exemplified by the large panel of R&D activities and projects ranging from the fundamental/laboratory level to industrial scale production mostly performed by reactive processing (particularly reactive extrusion, so-called REx). Additionally, to allow the rapid implementation of novel products, UMONS and Materia Nova have recently created NANO4 S.A., a spinoff company specialized in production, functionalization, characterization and processing of nanofillers, incl. renewable biosourced nanoparticles, and their related masterbatches. Accordingly, NANO4 S.A. allows for the up-scaling of new bio(nano)composites characterized by specific end-use properties such as gas barrier, flame retardancy (FR), UV absorption, antibacterial action, tailored electrical behavior, etc.

From Science & Research

Case study 1: PLA-gypsum-clay (nano)composites with specific flame retardant properties The traditional technology for the production of lactic acid (LA) leads in the formation of large amounts of hydrated calcium sulphate, i.e., for each kilogram of LA, about one kilogram of gypsum is formed as a by-product [4, 5]. In response to the demand for extending the range of PLA applications, while reducing production cost, it has been demonstrated that commercially available PLA can be effectively melt-blended with previously dehydrated gypsum (so-called CaSO4 β-anhydrite II (hereafter noted AII), thus the by-product directly issued from LA fabrication process [4]. For achieving high performance PLA composites and for preventing polyester chain degradation by hydrolysis, it is important to specifically use AII microparticles, which is actually formed by dehydration of gypsum hemihydrate at 500 °C. These two products (PLA and AII) from the same source as origin can lead by melt-mixing to polymer composites characterized by remarkable thermal stability, high rigidity, good tensile strength and barrier properties even at high AII content (up to 40 wt%). Such performances could be ascribed to the fine microfiller dispersion and good interfacial characteristics. Moreover, like for other mineral-filled polymers, addition of a third component into PLA–AII compositions, e.g., plasticizers, flame retardants, nanofillers, has been considered in order to generate new PLA grades with specific end-use performances. It was discovered (WO 2008/095874 A1 and US 2010/0184894 A1 patents: ‘Polylactide-based compositions’) that co-addition of dehydrated CaSO4 (AII form) and adequately selected organo-modified layered silicates (OMLS) triggers synergistic effects on PLA fire-resistant properties. [5, 6] Interestingly enough, the production of these ternary PLA-AII-OMLS bio(nano)composites, has been successfully conducted by melt-compounding in a co-rotating twin-screw extruder as illustrated in Figure 1. The different starting materials that were investigated are: PLA, was supplied by NatureWorks LLC as PLA 3051D (Mn(PS) = 112 000; Mw/Mn = 1.95; D-isomer = 4.3 %). Calcium sulphate hemihydrate, the by-product obtained from lactic acid production process (d50 of 9 μm) was provided by Galactic S.A. Starting from this filler, β-anhydrite II (AII) was obtained by drying at 500 °C for 1 h. A natural calcium sulphate anhydrite (USG CAS-20-4, d50 of 4 μm) kindly supplied by USG Company was also studied. This product was used only as alternative for gypsum from

350 RHR (kW/m2)

Two selected key-results, relying upon the original production of innovative bio(nano)composite materials using PLA as polyester matrix, with targeted applications in packaging, in textile fibers and in the field of engineering sector, are summarized hereinafter.

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PLA

PLA- AII - clay (nano(composites: Decrease of pRHR, higher ignition time ...

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PLAAII - clay

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— PLA — PLA- 40% AII (9) - 3% B104 — PLA- 40% AII (4) - 3% C10A

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(a)

Figure 2: RHR plotted against time: neat PLA compared to PLA- gypsum AII- clay (nano)composites (by courtesy, tests performed by Dr. Antoine Gallos –ENSC Lille)

lactic acid production process and as microfiller of lower dimensions. Bentone 104 (Elementis Specialties) and Cloisite 10A (Southern Clay Products, Inc.), two montmorillonite-type clays organo-modified with benzyl dimethyl hydrogenated tallowalkyl ammonium, respectively coined as B104 and C10A, were investigated as OMLS. Highly filled (nano)composites, i.e., PLA with 40 wt% in AII and 3 wt% in clay, were thus produced at semi-pilot scale in a twin-screw extruder (Leistritz type ZSE 18 HP-40D, Ø = 18mm, L/D = 40) and the so-produced granules were characterized using various techniques. Firstly, it is worth mentioning that the good thermo-mechanical performances, comparable to those of conventional filled engineering polymers, are ascribed to the excellent filler (AII and OMLS) dispersion throughout the polyester matrix as evidenced by electronic microscopy [4, 5]. By considering the high content in inorganics (e.g., 40% and 3% in micro- and nanofillers, respectively), these materials are characterized by good tensile strength (≈ 37 MPa), whereas the rigidity, i.e., Young’s modulus, is above 6300 MPa, that means an increase of 125% with respect to neat PLA (2800 MPa). Besides, as evidenced by thermogravimetry analysis (TGA) these (nano)composites are characterized by improved thermal stability (e.g., following as criterion the temperature for 5% weight loss- T5%), whereas DSC analyses attest for the preservation of principal thermal parameters with even some increase of the PLA crystallization rate, property that can be considered as very promising in the perspective of further applications. Remarkably, the co-addition of gypsum AII and OMLS largely improves the fire-resistance of PLA as evidenced by cone calorimetry testing (Figure 2). The time to ignition (tig) is increased and the peak of maximum rate of heat release (pRHR) is reduced by almost 50% with respect to neat PLA. In addition, the horizontal fire test UL94 HB reveals a low speed of burning (29-31 mm/min) - corresponding to


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From Science & Research PLA 3051D

PLA - 40% AII- 3% B104

A

B

C

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 CaSO4 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.

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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 (T5%) 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].

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From Science & Research Further prospects: PLA-based hybrid nanocomposites

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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 CaSO4 (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.

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Figure 5: UV-vis spectra of selected samples of PLA-ZnO (silane treated) films compared to neat PLA evidencing total anti-UV protection

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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 FNRSFRFC. 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.


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