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High-Performance Polylactide/ZnO Nanocomposites Designed for Films and Fibers with Special End-Use Properties Marius Murariu,*,† Awa Doumbia,‡ Leila Bonnaud,† AnneLaure Dechief,† Yoann Paint,† Manuela Ferreira,‡ Christine Campagne,‡ Eric Devaux,‡ and Philippe Dubois*,† †
Center of Innovation and Research in Materials & Polymers (CIRMAP), Laboratory of Polymeric and Composite Materials (LPCM), University of Mons & Materia Nova Research Center, Place du Parc 20, 7000 Mons, Belgium ‡ Universite Lille Nord de France, F-59000 Lille, France & Ecole Nationale Superieure des Arts et Industries Textiles (ENSAIT), Laboratoire de Genie et Materiaux Textiles (GEMTEX), 2 Allee Louise et Victor Champier, BP 30329, 59056 Roubaix Cedex1, France ABSTRACT: Metallic oxides have been successfully investigated for the recycling of polylactide (PLA) via catalyzed unzipping depolymerization allowing for the selective recovery of lactide monomer. In this contribution, a metallic oxide nanofiller, that is, ZnO, has been dispersed into PLA without detrimental polyester degradation yielding PLA/ZnO nanocomposites directly suitable for producing films and fibers. The nanocomposites were produced by melt-blending two different grades of PLA with untreated ZnO and surface-treated ZnO nanoparticles. The surface treatment by silanization proved to be necessary for avoiding the decrease in molecular weight and thermal and mechanical properties of the filled polyester matrix. Silane-treated ZnO nanoparticles yielded nanocomposites characterized by good mechanical performances (tensile strength in the interval from 55 to 65 MPa), improved thermal stability, and fine nanofiller dispersion, as evidenced by microscopy investigations. PLA/ZnO nanocomposites were further extruded in films and fibers, respectively, characterized by anti-UV and antibacterial properties.
1. INTRODUCTION In competition with petroleum-based polymers, polylactide or polylactic acid (PLA), is one of the most promising candidates for future developments; it is not only biodegradable but also is produced from non-fossil renewable natural resources by fermentation of polysaccharides or sugar, for example, extracted from corn or sugar beet (and corresponding wastes), therefore allowing the biological cycle to come full circle with PLA biodegradation as well as the photosynthesis process.13 Furthermore, PLA can be readily and quantitatively recycled by acid-catalyzed depolymerization into lactic acid via the socalled LOOPLA chemical recycling process.4 The market for biodegradable polymers is growing every year, and important demands can be expected for those applications where biodegradability offers a clear advantage for customers and the environment. PLA is currently receiving considerable attention for rather conventional applications such as packaging materials as well as production of textile fibers but also finds higher added value for technical applications, for example, when formulated with finely dispersed nanoparticles.3,58 Interestingly, PLA-based nanocomposites characterized by improved properties (stiffness, thermal stability, fire retardancy, lower permeability, etc.) have been produced by melt-blending PLA with different nanofillers such as organo-modified layered silicates (OMLS), carbon nanotubes (CNTs), expanded or exfoliated graphite, polyhedral oligomeric silsesquioxanes (POSS), and so on.914 r 2011 American Chemical Society
Among the different types of metal oxides that present high potential in fabrication of polymer nanocomposites, calcium oxide (CaO), magnesium oxide (MgO), and zinc oxide (ZnO) are known for their marked antibacterial activity even in low filler amount.15 Unfortunately, the addition to PLA of these metal oxides or other metallic compounds (e.g., layered double hydroxides) usually leads to intensive degradation of PLA chains resulting in a sharp reduction of the thermomechanical properties of the polyester matrix. Moreover, it is of interest to note that to achieve the feed stock recycling of PLA to lactide (LA) different “unzipping” depolymerization catalysts have been investigated with particular attention to alkali earth metal oxides, such as CaO and MgO.16,17 In relatively high percentage (5 wt %), these metal oxides lowered the degradation temperature range of PLA and completely suppressed the production of oligomers other than expected lactides.15 ZnO is a well-known environmentally friendly and multifunctional inorganic filler. ZnO is colorless, wide band gap semiconductor with an optical band gap in the UV region that makes it useful as an efficient absorber of UV radiation.18 Furthermore, ZnO nanofillers can be mixed with different polymers (polyamide-6, epoxy and acrylic resins, poly(methylmethacrylate), polystyrene, polypropylene, etc.) to produce nanocomposites Received: January 31, 2011 Revised: April 1, 2011 Published: April 06, 2011 1762
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Biomacromolecules characterized by a large array of properties such as effective antibacterial function, intensive ultraviolet absorption, or other required characteristic features.1922 Until now, ZnO as well as other Zn compounds have been successfully utilized as effective catalysts for LA polymerization but also in “unzipping” depolymerization of PLA and lactic acid oligomers (OLA).2325 In this context, it is reasonable to expect that the dispersion of ZnO nanofillers within PLA will trigger some intensive degradation of the polyester matrix, especially at high temperature, for example, along with melt-processing. In our opinion, the catalytic depolymerization ability of Zn-based products, including ZnO nanoparticles, likely represents the main reason explaining the lack of studies concerning the fabrication of PLA/ZnO nanocomposites. This contribution aims at studying the preparation of PLA nanocomposites filled with ZnO nanofillers, products potentially interesting in production of films and fibers designed with special end-use properties such as anti-UV and antibacterial effects. In this main objective, two PLA grades, for fibers and films, have been mixed by melt-compounding with surface-treated or untreated ZnO (commercially available grades) and the resulting nanocomposites fully characterized. Furthermore, in a second step, selected nanocomposites have been extrapolated on a semipilot scale to produce PLA/ZnO nanocomposite filaments and films characterized by multifunctional properties.
2. EXPERIMENTAL SECTION 2.1. Materials. Poly(L,L-lactide) grades were kindly supplied by NatureWorks LLC: a grade for films (Mn(PLA) = 88 500, index of polydispersity, Mw/Mn = 1.8, D isomer 10 °C).
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Specimens for tensile and Izod impact tests were obtained from plates by using a milling-machine in accordance with ASTM D 638-02a norm (specimens type V) and ASTM D 256-A norm (specimens 60 10 3 mm3). 2.2.1. Films. To allow the upscaling in realization of films, we produced higher quantities of PLA1/ZnO nanocomposites by meltcompounding PLA1 with up to 3% ZnO(s) nanofiller in twin-screw extruders (e.g., Leistritz-type ZSE 18 HP-40D, Ø = 18 mm, L/D = 40). The previously dried granules of PLA and additives were first mixed in a turbo-mixer (2000 rpm, 2 min) step, followed by the dosing and meltcompounding into the twin-screw extruder (conditions of processing: throughput up to 1.5 kg/h, speed of screws = 100 rpm, temperatures of extrusion adapted to the rheological characteristics of PLA, the temperatures of the molten polymers were ∼185 °C). After granulating, the granules of PLA1 nanocomposites were dried and used for the realization of films by extrusion. The drying of granules (overnight, at 80 °C, under vacuum) was followed by the production of 100150 μm thick films using a DSM twin-screw microextruder (batch-volume: 15 mL, speed of screws: 70 rpm, temperature of molten polymer: 185190 °C) equipped with a special die (width 35 mm, die opening: 0.4 mm) and a DSM Xplore microfilm device (speed: 200 mm/min, torque of winding unit: 4050 N mm). 2.2.2. Fibers. To process multifilament yarns, we prepared nanocomposites by using a conventional twin-screw extrusion device. PLA pellets with appropriate content of ZnO were blended using a corotating twin screw extruder (Thermo Haake, screw diameter = 16 mm, L/D = 25). The temperatures of the five heating zones were, respectively, 130/160/ 165/170/180 °C for PLA containing 3% ZnO. The rotation speed of the screws was maintained at 100 rpm for all experiments. After meltblending, the nanocomposite extrudate was pelletized prior to spinning. PLA and PLA/ZnO nanocomposites were spin-drawn with the melt spinning pilot Spinboy I designed by Busschaert Engineering. The pellets were molten in a single screw extruder with the following temperature profile: 195/205/210/200/185 °C. Then, the molten matrix was brought through two parallel spinning dies heated at 175 °C, consisting of 40 channels with a diameter of 400 μm each to obtain a continuous multifilament yarn. After extrusion, the multifilament yarn was coated with a spin finish CROSANOL I-PA07 to ensure its cohesion and to dissipate the static electricity generated during the processing. At last, the yarn was hot drawn between two rolls with different rotation speeds. The draw ratio (DR), defined as the ratio between the rotation speeds of the drawing and the feeding rolls (200 m/min), was optimized at 2.5, whereas the temperature of the feeding roll was fixed at 70 °C, and the temperature of the drawing roll was fixed at 90 °C. Multifilament yarns made with PLA and PLA nanocomposites were then knitted manually on a rectilinear machine gauge 5. The asproduced fabrics were used for antibacterial activities tests. 2.3. Characterization. 2.3.1. Size Exclusion Chromatography (SEC). Recovery of PLA from selected compositions for molecular weight parameters determination was carried out by first dissolving the samples in chloroform and following a similar procedure used in the past in the case of PLA/OMLS nanocomposites.9 The metallic residues were removed by liquidliquid extraction with a 0.1 N HCl aqueous solution step, followed by intensively washing with demineralized water. Finally, PLA was recovered by precipitation in an excess of heptane. After filtration and drying, PLA solutions were prepared in THF (10 mg polymer/5 mL solvent). Molecular weight parameters (number-average molar mass, Mn, and polydispersity index, Mw/Mn) of pristine PLA and PLA extracted from the studied nanocomposites were determined by SEC using the procedure and relations described elsewhere.9 2.3.2. Thermogravimetric Analyses (TGA). TGAs were performed by using a TGA Q50 (TA Instruments) with a heating ramp of 20 °C/min under air flow, from room temperature to 600 °C (platinum pan, 1763
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60 cm3/min air flow rate). Isothermal tests, as simulated by TGA, were performed by using HiRes TGA 2950 (TA Instruments) (platinum pans, air flow rate of 110 cm3/min). Procedure: heating ramp by 20 °C/ min from room temperature up to the desired temperature, followed by an isotherm for 30 min. The thermal analyses have been typically performed on samples issued from the compression molding process. 2.3.3. Differential Scanning Calorimetry (DSC). DSC measurements were performed using a DSC Q200 from TA Instruments under nitrogen flow. The samples obtained by compression molding were typically investigated. The procedure was as follows: first, heating scan at 10 °C/min from 0 to 200 °C, isotherm at this temperature for 2 min, then, cooling at 10 °C/min to 20 °C, and finally, second heating scan from 20 to 200 °C at 10 °C/min. The first scan was meant to erase the anterior thermal history of the samples. The events of interest, that is, the glass-transition temperature (Tg), cold crystallization temperature (Tc), enthalpy of cold crystallization (ΔHc), melting temperature (Tm), and melting enthalpy (ΔHm) were determined from the second scan. The degree of crystallinity was determined by subtracting ΔHc from ΔHm and by considering a melting enthalpy of 93 J/g for 100% crystalline PLA. 2.3.4. Mechanical Testing Measurements. Tensile testing measurements were performed by using a Lloyd LR 10K tensile bench in accordance with the ASTM D 638-02a norm at a speed rate of 1 mm/ min using a distance of 25.4 mm between grips. Notched impact strength (Izod) measurements were performed by using a Ray-Ran 2500 pendulum impact tester and a Ray-Ran 1900 notching apparatus in accordance with the ASTM D 256 norm (method A, 3.46 m/s impact speed, 0.668 kg hammer). All mechanical tests were carried out by using specimens previously conditioned for at least 48 h at 20 ( 2 °C under a relative humidity of 50 ( 3%, and the values were averaged out over five measurements. 2.3.5. Transmission Electron Microscopy (TEM). Transmission electron micrographs were obtained with a Philips CM200 apparatus using an accelerator voltage of up to 120 kV. The samples (7080 nm thick) were prepared with a Leica Ultracut UCT ultracryomicrotome by cutting at 100 °C. Reported microphotographs represent typical morphologies, as observed at, at least, three various places. 2.3.6. UV Absorption Properties. UVvisible absorption spectra were measured on films of 100300 μm thickness (prepared by compression molding or by extrusion) using a UVvis Varian Cary 5G spectrophotometer. The blank reference was air. Transmittance spectra were recorded in the 200800 nm wavelength range. The spectrum of neat PLA was compared with those of nanocomposites, and the average of minimum three samples was considered. 2.3.7. Antibacterial Properties. The standard NF EN ISO 20743 was used to conduct antibacterial testing. It is a standard test for antibacterial activity assessment on textile products. All samples were washed following NF EN ISO 6330 standard to remove the spin finish prior to bacteria inoculation. Samples were seeded with Staphylococcus aureus or Klebsiella pneumoniae bacteria. The quantitative test method used involved the direct counting of bacteria before and after incubation at 37 °C for 24 h. The obtained values were used to calculate the antibacterial activity (A) following the formula A ¼ FG With F = (log C24 log C0) and G = (log T24 log T0), which represent, respectively, the growth value of the untreated (i.e., control) PLA sample and treated sample (PLA/ZnO nanocomposite). C and T represent the number of bacteria counted from untreated and treated textiles, respectively, whereas “24” and “0” account for the time of incubation in hours (h). The products are considered to be “antimicrobial” when achieving an activity (A) superior to 2.0 (reduction in bacteria number >99%).
3. RESULTS AND DISCUSSION In general, the addition of micro- or nanofillers provides PLA with specific properties but sometimes can trigger problems such as loss of mechanical and thermal properties due to intensive degradation of the polyester matrix, aspects that need to be considered when targeting potential applications such as production of fibers or films. The first objective of the study is to show how much the addition of different ZnO nanofillers (surface-treated or not) to PLA will affect the molecular parameters and properties of the polyester matrix. The second objective is to explore the most interesting PLA/ZnO nanocomposites in the production of films and fibers designed with specific end-use properties such as antiUV and antibacterial protection. To make PLA matrix less susceptible to the catalytic action of ZnO during the melt blending process and subsequent film/fiber processing, various filler surface treatments, with different effectiveness (results not discussed here), have been considered: physical coating with stearic acid or stearates, treatment with various copolymers and plasticizers, and chemical coupling with silanes. It is well known that incorporation of coupling agents such as silanes onto the filler surface is an efficient way of modifying the polymer matrixfiller interface.26 The versatility of the silane chemical structure allows us to combine the function of dispersing and sometimes of coupling agent so they are capable of either linking the filler and the polymer matrix by chemical bonds or simply rendering the filler more hydrophobic and more easily dispersible in organic phase. Minerals surface-covered with hydroxyl such as ZnO nanoparticles are generally very receptive to bonding with alkoxysilanes.27 Silanol groups formed as a result of a hydrolysis reaction react with hydroxyl groups found on ZnO surfaces to form siloxane bonds through condensation reactions. Moreover, silane molecules also react with each other to generate a multimolecular structure of silane coupling agents anchored onto the nanofiller surface. More than one (mono)layer of silanes is usually applied to the surface of the filler, resulting in a tight siloxane network close to the inorganic surface. 3.1. PLA/ZnO Nanocomposites: Characterization of Thermal and Molecular Properties. In the perspective of melt-
mixing with PLA, it is important to point out that by comparing with OMLS, which generally show the onset of thermal degradation even at a temperature lower than 200 °C,28 ZnO nanofillers are characterized by an excellent thermal stability. In fact, according to TGA, except for some residual moisture, untreated ZnO does not show any important decrease in weight up to 600 °C, whereas because of the treatment with silanes, that is, triethoxy caprylylsilane (Experimental Section), a weight loss of 2.6% was found at the temperature of 500 °C for the surfacetreated ZnO(s) grade. Concerning the PLA matrices, the two studied grades show nearly comparable degradation intervals in the temperature range 315400 °C with PLA1, that is, the polyester of higher molecular weight, characterized by slightly better thermal stability (Table 1). In relation to the thermal stability of polymer nanocomposites, as reported elsewhere, the addition of ZnO to various polymer matrices (e.g., PMMA29) can lead to either stabilizing or degradation effects. Like in the case of OMLS, the preparation method can also influence the thermal stability; frequently, a more advanced nanofiller dispersion is associated with nanocomposites showing higher temperature of degradation.30 To examine the comparative effects of ZnO and ZnO(s) addition on 1764
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Table 1. Thermal Properties of PLA1 and of PLA2 with Different Loadings of Surface-Treated or Untreated ZnO Nanofiller sample composition V (wt %) entry
temperature for 5% weight loss (°C)
nanofiller type f
ZnO
ZnO(s)
temperature of the maximum rate of degradation (°C) (from D-TG)
337
ZnO
ZnO(s)
1
PLA1 (0% ZnO)
2
PLA1 - 1% ZnO
266
289
381 313
339
3
PLA1 - 2% ZnO
256
285
304
332
4
PLA1 - 3% ZnO
252
275
299
327
5
PLA2 (0% ZnO)
6
PLA2 - 1% ZnO
253
283
301
336
7
PLA2 - 2% ZnO
251
285
297
338
8
PLA2 - 3% ZnO
241
277
294
322
328
368
Scheme 1. Transesterification (a) and “Unzipping” Depolymerization (b) Reactions of PLA in the Presence of Zn Compounds (Adapted from Ref 23)
the thermal stability of PLA1 and PLA2, we have compared TG measurements on nanocomposite samples with those of pristine PLA. For simplification and easier interpretation, Table 1 shows the values of the temperature for 5% weight loss (T5%) and of the temperature corresponding to the maximum rate of thermal degradation (TD - from D-TG). It is of interest to mention that T5% is often considered to be the initial decomposition temperature.31 From the TGA results shown in Table 1, it comes out that on one hand, for both PLA matrices, the addition of untreated ZnO nanofiller leads to a more intensive decrease in T5% and TD with respect to the neat matrix. As an illustration, the addition of 3% ZnO triggers a decrease in T5% by >85 °C. Moreover, it can be seen that the reduction in thermal stability is in direct correlation with the percentage of nanofiller, whereas PLA of higher molecular mass (PLA1) shows somewhat better thermal parameters than PLA2. It is noteworthy to mention that in all cases the corresponding nanocomposites obtained using ZnO(s), that is, surface-treated ZnO, are characterized by better thermal performances than those obtained using untreated ZnO, and with few exceptions, there are similar tendencies regarding the decrease in T5% or TD
values with the increase in nanofiller content. For instance, PLA/ ZnO(s) nanocomposites (with PLA1 or PLA2 as matrix) show T5% and TD values recorded at significantly higher temperature, from 20 to 40 °C, with respect to the samples containing untreated nanofiller. Such improvements can be considered to be of real interest in the perspective of applying these nanocomposites in the production of films or fibers. In another context, for comparative evaluation and the further selection of polymer nanocomposites, it is of real importance to have information about their thermal stability at temperatures comparable to those of processing or even at higher temperature. It is noteworthy to mention that in previous studies23 it was reported that under isothermal conditions (e.g., at 220 and 255 °C) zinc compounds catalyze both the intermolecular transesterification reactions, generating PLA with lower molecular weights, and the “unzipping” depolymerization, leading finally to selective formation of LA (as represented in Scheme 1). Indeed, it is worth pointing out that parallel studies carried out by some of us in the main goal of chemical recycling of PLA have confirmed the effectiveness of ZnO in catalyzing the “unzipping” depolymerization of PLA. Isothermal TGA performed at 230 °C, that is, a temperature higher than those generally used for PLA 1765
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Figure 1. (a,b) TGA under isothermal conditions (under air) of PLA1 with different loadings of surface-treated or untreated ZnO at (a) 200 and (b) 220 °C.
processing, on PLA samples containing up to 6% ZnO nanoparticles gave evidence of a marked loss in weight strongly dependent on the nanofiller content. Moreover, and in perfect agreement with previously reported data,23 the volatile products formed all along the isothermal degradation have been characterized by TGA coupled to FTIR technique (spectra not shown here), attesting to the formation of essentially LA cyclic monomer as well as some traces of oligomers (OLA). Figures 1a,b and 2a,b show the comparative TG measurements performed under isothermal conditions at 200 and 220 °C for both PLA1 and PLA2 filled with increasing contents of surface-treated ZnO(s) or untreated ZnO nanoparticles. It comes out that a significantly better thermal stability is observed for the nanocomposites containing ZnO(s), whereas the increase in weight loss is connected to additional parameters such as nanofiller content, temperature, and residence time. For illustration, the weight loss recorded after a residence time of 30 min at 220 °C for PLA1 and PLA2 samples containing 3% untreated ZnO reached values as high as 31.7 and 43.7%, respectively. Contrarily, under similar conditions of analysis, the nanocomposites containing 3% ZnO(s) proved to be obviously more stable, with weight loss limited to only 7.7 and 9.9%, respectively. Because the anti-UV properties can be obtained with contents in ZnO(s) as low as 1% or less (Section 3.3), it is assumed that because of their improved thermal stability, these PLA/ZnO(s) nanocomposites could be of high interest for the production of films and fibers.
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Figure 2. (a,b) TGA under isothermal conditions (under air) of PLA2 with different loadings of surface-treated or untreated ZnO at (a) 200 and (b) 220 °C.
In relation to the thermal properties, it is also of interest to note that the DSC characterizations did not evidence in the case of nanocomposites containing ZnO(s) any noticeable changes of the principal parameters such as glass transition (Tg) and melting temperature (Tm). The addition of surface-treated nanofiller does not modify the Tg of the polyester matrix (neat PLAs show the Tg at 62 °C, in nanocomposites it was revealed at 6163 °C), whereas the Tm was kept in nanocomposites in similar interval (161169 °C) like for PLAs (peaks of fusion in the range 163168 °C). In general, the PLA samples show double peaks of fusion that are ascribed to the melting of crystalline regions of various size and perfection formed during cooling (after the first DSC heating scan) and cold-crystallization (during second DSC heating scan), processes that were specific for both neat PLA and nanocomposites. In the same context, it is noteworthy to mention that the addition of untreated ZnO has as effect a slight decrease in Tg (values in the range 5760 °C) and in Tm (double peaks in the interval 156168 °C); these modifications are ascribed to the formation of LA or OLA produced by the degradation of PLA matrices. Simply the addition of ZnO nanofillers, surface-treated or not, allows for slightly increasing the crystallinity degree of the PLA matrix. The increases in the degree of crystallinity (the neat PLAs show low values, i.e., 12.5%) were more important for the nanocomposites obtained using surface-treated filler (values up to 10%) and PLA1 as matrix. (Details concerning the DSC results will be presented in a forthcoming communication.) 1766
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The modifications of the molecular weights of the polyester matrix in nanocomposites have also been studied by SEC. First of all, it is worth recalling that PLA is very sensitive to temperature, shear, and hydrolysis during melt-processing.31,32 The degradation of PLA is usually attributed to hydrolysis by trace amounts of water, zipper-like depolymerization, thermo-oxidative reactions, and random main-chain scission during melt-processing, which can be catalyzed by residual acidity and the presence of different metal impurities. In the specific case of PLA/ZnO nanocomposites, one can expect some catalytic behavior of metallic ions of Zn on the degradation of PLA during meltmixing and processing. Figure 3a,b shows the modification of PLA molecular characteristics (Mn and PI) after melt-blending and compression molding of the samples containing up to 3% surface-treated or untreated ZnO. First, in the absence of nanofiller, neat PLA1 shows some reduction of Mn, that is, from 88 500 (as granules) to ∼67 000 after melt-processing. PLA1 seems to be slightly more sensitive to degradation under mechanical shearing than neat
Figure 3. (a,b) Modification of molecular parameters (Mn (a) and PI (b)) of PLA1 and PLA2 with different loadings of surface-treated or untreated ZnO.
PLA2, which does not show significant modifications of molecular weight after processing (initial Mn = 55 000), at least within experimental errors, which can be estimated to be 15% taking into account SEC accuracy but also PLA extraction and purification steps. (See the Experimental Section). The addition of untreated nanofiller leads to a more pronounced decrease in Mn (Figure 3 a) and a concomitant increase in PI (Figure 3 b). From these results, even in the presence of surface-treated ZnO(s), it was not possible to completely avoid the degradation of the polyester matrix. Interestingly enough, in the presence of ZnO(s), the molecular weights remain high enough for offering nanocomposites with very acceptable mechanical properties (see section 3.2, mechanical characterization). Clearly, the surface-coating of ZnO nanofiller allows for limiting the decrease in PLA molecular weights and preserving good thermal stability of the resulting PLA, improvements that might be ascribed to the shielding effect conferred by the organo-silane layers (ZnOSiR (R = caprylyl)) and SiOSiO network formation). 3.2. PLA/ZnO Nanocomposites: Mechanical Properties and Morphology. Table 2 and Figure 4a,b (maximum tensile strength (a) and (b) Young’s modulus) gather the mechanical
Figure 4. (a,b). Evolution of tensile strength (a) and Young’s modulus (b) of PLA1 with different loadings of surface-treated or untreated ZnO.
Table 2. Comparative Mechanical Properties of PLA2 and PLA2ZnO(s) Nanocomposites (Standard Deviations Are Given in Brackets) entry
a
sample (%, by weight)
max. tensile strength (MPa)
Young’s modulus (MPa)
nominal strain at break (%)a
impact strength Izod (kJ/m2)
1
PLA2 (0% ZnO(s))
61 ((2)
2100 ((150)
6.3 ((0.5)
2.7 ((0.2)
2 3
PLA2 - 1% ZnO(s) PLA2 - 2% ZnO(s)
58 ((1) 59 ((4)
2250 ((100) 2150 ((150)
4.4 ((0.2) 5.2 ((0.3)
2.8 ((0.2) 2.7 ((0.2)
4
PLA2 - 3% ZnO(s))
57 ((3)
2600 ((150)
3.7 ((0.2)
2.9 ((0.4)
Distance between grips of 25.4 mm. 1767
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Figure 5. (ad) TEM pictures at different magnifications of PLA2 nanocomposites containing 2 wt % untreated (a,b) and surface-treated ZnO (c,d).
properties of neat PLAs compared with nanocomposites containing increasing amounts of nanofiller, surface-treated or not. First of all, it comes out that the interface between the ZnO nanoparticles and the polyester matrix plays a key-role in the structureproperty relationship. On one hand, as seen in Figure 4 a, a strong decrease in tensile strength of ∼50% was recorded in the case of nanocomposites obtained by the addition of 2 to 3% untreated nanofiller (ZnO) to PLA1 (neat PLA1 has a tensile strength of 61 MPa). Moreover, the mechanical characterization of the nanocomposites having PLA2 as matrix (PLA of lower molecular mass) and similar loadings of untreated nanofiller was not possible (too brittle samples). The decrease in performance especially for the polyester matrix of low molecular weight (PLA2) was not surprising taking into account the results of thermal and molecular characterizations. As seen in Figure 4 a and Table 2, the addition of surfacetreated nanofiller with silane leads to an important increase in the
tensile strength with respect to nanocomposites obtained with the untreated ZnO nanofiller. Whatever the nature of the PLA matrix, nanocomposites filled from 1 to 3% surface-treated ZnO(s) show good mechanical properties, that is, a tensile strength in the range of 5565 MPa, values that are comparable to or somewhat higher than those obtained for the neat polyester matrix. The increase in the stress values of the nanocomposites indicates that the PLA/ZnO interface is tuned by treatment with the silane surface-agent. These improvements are generally associated with lower interfacial energy between the polymer matrix and nanofiller as well as finer dispersion of the nanoparticles. Furthermore one cannot exclude the effect of the Si OSiO layers that cover the surface of nanofiller and behave as a barrier effectively limiting the catalytic effect of ZnO and allowing for reducing the extent of unzipping/transesterification reactions as aforementioned. The Young’s modulus increases with the relative content in nanofiller (ZnO or ZnO(s)) (Figure 4 b), whereas the nominal 1768
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Figure 6. (a,b) PLAZnO nanocomposites tested in production of films (a) and fibers (b).
strain at break decreases, as shown by the addition of ZnO(s), for example, from 6.3% (neat PLA2) to 3.7% (PLA2 - 3% ZnO(s)), (Table 2). Concerning the notched impact strength (Izod) of the nanocomposites (Table 2, PLA2 as matrix), the nanocomposites are characterized by values comparable to those recorded for neat PLA (2.7 kJ/m2). Finally, as a preliminary conclusion, the mechanical characterization confirms that dispersion of surface-treated ZnO(s) allows for preserving very acceptable mechanical properties of the related nanocomposite materials, whereas a sharp reduction is recorded by the addition of untreated ZnO, particularly in PLA2 as matrix. The morphology of PLA/ZnO nanocomposites was obtained from TEM images. Figure 5ad shows selected TEM images of PLA2 - 2% surface-treated/untreated ZnO nanocomposites. It is obvious from the TEM images that the surface coating by the silane agent triggers finer dispersion and distribution of the ZnO(s) nanoparticles. Indeed, without any surface-treatment, some clusters/aggregates of ZnO nanofillers remain present throughout the polyester matrix (Figure 5 b). Noteworthy, similar results have been obtained using PLA1 as matrix (images TEM not shown here). 3.3. PLA/ZnO Nanocomposites for Fibers and Films with Specific End-Use Properties. Following the preliminary results, different PLA/ZnO and PLA/ZnO(s) nanocomposites have been further produced on a larger scale via corotating twin-screw extrusion to obtain granules that were successfully used to produce films and fibers (Figures 6a,b) characterized by specific end-use properties. 3.3.1. UV Absorption Properties on Films. PLA/ZnO(s) nanocomposite films of 100300 μm thickness have been prepared by compression molding (Experimental Section) or following
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Figure 7. (a,b) UVvis spectra on selected samples of PLA1ZnO(s) and PLA2ZnO(s) compared with the neat PLA matrices.
the upscaling step by extrusion using a DSM film device equipment. It clearly appears that the resulting films are characterized by effective anti-UV protection. For illustration, Figure 7a,b shows the UVvis spectra of PLA1 and PLA2ZnO(s) nanocomposites with different loading of nanofiller. From these results, it comes out that in both polymer matrices the addition of 1% ZnO(s) or less leads to PLA nanocomposites showing an almost perfect UV light shielding efficiency while maintaining rather good transmittance in the visible range of the spectra. 3.3.2. Antibacterial Properties. Representative nanocomposite fibers with a content of 3% of untreated ZnO have been meltspun with the objective to evaluate the antibacterial activity of ZnO dispersed in PLA matrix. (See the Experimental Section.) The spinning dies were heated to 175 °C and thus ca. 10 °C above the melting temperature of pristine PLA (PLA2 as matrix, spinning grade). Interestingly enough, the so-recovered nanocomposite yarns displayed mechanical properties adapted for producing knitted fabrics. The nanocomposite fabrics (as well as those of neat PLA) have been knitted manually and used for the evaluation of their antibacterial properties. Both Gram positive and Gram negative bacteria have been tested, and the results of antibacterial analyses are given in Tables 3 and 4. All values are given as a logarithm (to base 10) and counted by colony forming unit (CFU) for each sample. The reported results are the average values of three samples. The antibacterial activity (A) corresponds to the difference between growth values of untreated (PLA) and treated samples (PLA/3%ZnO). The growth values recorded on neat PLA fabrics (control sample) are positive for the two types of tested bacteria (Gram 1769
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Table 3. Results of Antibacterial Properties of Knitted Fabrics (PLA and PLA/3% ZnO) toward Staphylococcus aureus Gram Positive Bacteria average (log CFU) sample
antibacterial activity (A)
0h
24 h
growth
PLA
4.69
6.90
F = 2.21
(A = F G)
PLA/3% ZnO
4.66
2.57
G = 2.09
A = 4.3
PLA PLA/3% ZnO
4.66 4.70
*Phone: þ3265554976. Fax: þ32655549703. E-mail: marius.
[email protected] (M.M),
[email protected]. be (P.D.).
24 h
growth
activity (A)
7.63 1.00
F = 2.97 G = 3.70
(A = F G) A = 6.67
’ REFERENCES
average (log CFU) 0h
Corresponding Author
’ ACKNOWLEDGMENT We thank the Wallonia Region, Nord-Pas de Calais Region and European Community for the financial support in the frame of the IINTERREG IV - NANOLAC project. They thank all partners, especially Professor Serge Bourbigot (ENSC Lille) and his collaborators, for helpful discussions and all mentioned companies for supplying raw materials. This work was also supported by the European Commission and Region Wallonne FEDER program (Materia Nova) and OPTI2MAT program of excellence by the Interuniversity Attraction Pole program of the Belgian Federal Science Policy Office (PAI 6/27) and by FNRSFRFC.
Table 4. Results of Antibacterial Properties of Knitted Fabrics (PLA and PLA/3% ZnO) toward Klebsiella pneumoniae Gram Negative Bacteria
sample
’ AUTHOR INFORMATION
antibacterial
positive and Gram negative). This means that bacteria growth occurs during incubation time, attesting for the absence of any antibacterial effect on PLA fabrics. As far as the PLA nanocomposite fabrics are concerned, the growth values proved negative whatever the type of bacteria. Indeed, the bacteria population substantially decreased all along the incubation time, clearly evidencing the antibacterial effect of ZnO nanoparticles dispersed within the PLA matrix, leading to excellent antibacterial activities, that is, with A value higher than 4.
4. CONCLUSIONS On one side, according to the literature, ZnO and Zn compounds are known to catalyze “unzipping” depolymerization of PLA. On the other side, this work demonstrates that it is possible to obtain competitive nanocomposites using commercially ZnO nanofillers and PLA, particularly when the polyester/ nanofiller interface is adequately tuned, that is, via silane surfacetreatment. To show how much the addition of ZnO will affect the properties of the polyester matrix, two grades of PLA matrices and up to 3% surface-treated or untreated ZnO have been mixed by melt-compounding to produce nanocomposites. The study revealed that addition of untreated ZnO to PLA leads to some loss of the thermomechanical performances. The important decrease in properties, in direct correlation with the nanofiller loading, is mainly ascribed to the decrease in PLA molecular weights. Contrary to the untreated nanofiller, surface-treated ZnO(s) by triethoxy caprylylsilane is leading to nanocomposites characterized by noticeable thermomechanical performances (e.g., tensile strength in the interval from 55 to 65 MPa), whereas good dispersion/ distribution of ZnO(s) on a nanoscale level was revealed by TEM. Moreover, PLA/ZnO(s) nanocomposite films could be further produced via extrusion processing attesting for their very effective anti-UV action even at low amount of ZnO(s), as tiny as 1%. Interestingly enough, PLA/ZnO nanocomposite fibers have also been melt-spun, and their antibacterial protection has been evidenced.
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