From Science & Research
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 Drying all components (1) Gypsum AII + clays (dry-mixing) (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 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. 46 bioplastics MAGAZINE [01/12] Vol. 7
From Science & Research 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. 350 300 250 200 RHR (kW/m 2 ) PLA PLA- AII - clay (nano(composites: Decrease of pRHR, higher ignition time ... 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 CaSO 4 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 CaSO 4 (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 (M n(PS) = 112 000; M w /M n = 1.95; D-isomer = 4.3 %). • Calcium sulphate hemihydrate, the by-product obtained from lactic acid production process (d 50 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, d 50 of 4 μm) kindly supplied by USG Company was also studied. This product was used only as alternative for gypsum from 150 100 50 PLA- AII - clay 0 0 100 200 300 400 500 600 700 — PLA — PLA- 40% AII (9) - 3% B104 — PLA- 40% AII (4) - 3% C10A 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 nano- fillers, 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- T 5% ), 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 (t ig ) 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 (a) Time (s) bioplastics MAGAZINE [01/12] Vol. 7 47
