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clay nanocomposites: effect of nature and content of clay on morphology, thermal and thermo-mechanical properties
Materials Science and Engineering C 32 (2012) 1790–1795
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Poly(lactic acid)/clay nanocomposites: effect of nature and content of clay on morphology, thermal and thermo-mechanical properties Kikku Fukushima a,⁎, Daniela Tabuani b, Giovanni Camino a a b
Politecnico di Torino sede di Alessandria, INSTM research unit, Viale Teresa Michel 5, 15100 Alessandria,Italy PROPLAST Consortium, Viale Teresa Michel 5, 15100 Alessandria, Italy
a r t i c l e
i n f o
Article history: Received 22 August 2011 Received in revised form 5 March 2012 Accepted 22 April 2012 Available online 30 April 2012 Keywords: PLA Nanocomposites Layered silicates Sepiolite Thermo-mechanical properties
a b s t r a c t Nanocomposites were prepared by melt blending Poly(lactic acid) with 5 and 7 wt% of an organically modified montmorillonite or an organically modified magnesium sodium fluoro-hectorite or unmodified sepiolite. All nanocomposites show a good level of clay dispersion into the polymer matrix as well as a considerable thermal and thermo-mechanical properties improvement. According to thermal analysis, the clays seem to act as nucleating agents inducing a higher degree of crystallinity of the polymer and rate of crystallization. Similar increases in the thermal stability of Poly(lactic acid) were obtained for all clays. Concerning layered silicate nanocomposites, it was found that the main influencing factors on the thermo-mechanical properties appear to be the aspect ratio and dispersion of clay nanoplatelets, rather than polymer/clay chemical compatibility. Needle like sepiolite shows thermo-mechanical improvements comparable to some layered silicates and an interesting ability to maintain high storage modulus values at increasing temperatures, due to its good dispersion within the polymer without the need of organic modifiers as instead necessary for layered clays used in this work. © 2012 Elsevier B.V. All rights reserved.
1. Introduction During the past few decades, poly(lactic acid) (PLA) has attracted much attention for ecological, biomedical and pharmaceutical applications because it can be produced from renewable resources such as starch, it has good mechanical properties, it is biodegradable in the human body as well as in natural environments and the toxicity of its degradation products (lactic acid and oligomers) is very low [1–3]. Even more, as far as the biodegradability of PLA is concerned, it has been reported that the resulting products of the PLA hydrolytic degradation can be totally assimilated by microorganisms such as fungi or bacteria [4–7]; for example a PLA-degrading microorganism, Amycolatopsis sp., was recently isolated and characterized [5]. Other reports have suggested that enzymes have a significant role in PLA degradation [8–10]. In this way, Hoshino and Isono [10] have reported that PLA can be completely degraded at 55 °C, pH 8.5 in 20 days by the commercial lipase PL, derived from Alcaligenes sp. They concluded that complete degradation of PLA resulted from two processes: Firstly, the chemical hydrolysis from PLA into oligomers at higher pH and/or under higher temperature conditions, because polyesters are generally not stable under such conditions. Secondly, the enzymatic hydrolysis from the oligomers to the monomer.
⁎ Corresponding author. E-mail address: [email protected] (K. Fukushima). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.04.047
The main limitations of PLA towards its industrial application are its poor thermal and mechanical resistance, which limit its access to industrial sectors in which its use would be justified when biodegradability is required [3,11]. The above drawbacks could be overcome by enhancing the thermo-mechanical properties of PLA through copolymerization, blending and filling techniques. Indeed, the addition of nano-sized fillers in biodegradable polymers has a strong potential for designing eco-friendly materials for several applications, as the efficiency of nanofillers can be significant thanks to their high aspect ratio [3,12–15]. Among the most used nanofillers, layered silicates have been the most studied and were found to improve, even at low filler content (1–5 wt%), the mechanical, thermal, barrier and flame retardance properties of polymers, in comparison to unfilled matrices and to conventional microcomposites [13,16,17]. As far as biodegradable polymers are concerned, this route has been explored by Ray et al. [18–21], who prepared a series of PLA with various types of organically modified layered silicates, achieving important improvements in thermal and mechanical properties. In a subsequent research, the same group [22] prepared PLA nanocomposites with organically modified synthetic fluoromica, finding that PLA chains were highly intercalated within the clay. In parallel, the design of new nano-biocomposites should contain an assessment of the ecotoxicity of the degradation products after fragmentation, to ensure the effective assimilation by microorganisms without any hazardous effects on them and the environment [23]. Until now, the knowledge about the environmental and toxicological
K. Fukushima et al. / Materials Science and Engineering C 32 (2012) 1790–1795
effects of these newly produced nanomaterials is limited; it is expected, though, that these implications will be completely specific to the type of base material, and its size and shape. Generally, the addition of nanoclays in polymer matrices has been widely reported in the literature to improve the biodegradation of the resulting nanocomposites [24,25], although the exact mechanisms are not well known yet. This behaviour has been generally attributed to the high relative hydrophilicity of the clays, allowing an easier permeability of water into the material, thus accelerating the hydrolytic degradation process of the polymer. Moreover, the different pHs that arise in the proximities of the nanoclays after water absorption may catalyse the enzymatic activity of depolymerase and contribute to a remarkable enhancement of the biodegradation rate in nanocomposites [23], as it has been demonstrated in biodegradation studies using enzymes, compost and fungi media [26]. Preliminary toxicological tests in compost and by an isolated bacterium from the same compost on PLA reinforced with the organically modified layered silicates used in this work [27–29], have indicated that the biodegradation of these materials seems to be ecologically safe, observing no mortality in the bacterial colonies after nanocomposite incubation. In a previous work of ours [30] we have evidenced the possibility of sepiolite to be effectively dispersed without the need of organic modifiers, due to its large quantity of silanol groups located on surface and edges of each single needle [31], which could significantly improve its compatibility with polymer matrix. The aim of this work is to evaluate the effect of three nanoparticles with different chemical and physical structure on the thermal and thermo-mechanical properties of PLA, which is one of the most promising biodegradable polymers for a variety of industrial applications.
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mixer (Rheomix-Brabender OHG 47055) with a mixing time of 5 min, at 165 °C. The mixing was performed at two different rotor speeds: 30 rpm in the loading step and 60 rpm during mixing. The batch was extracted from the mixing chamber manually, allowed to cool to room temperature in air and ground in a rotatory mill. Sheets were obtained by compression molding in a hot-plate hydraulic press at 210 °C followed by cooling at room temperature under pressure. Characterizations were made on 0.5 mm films. 2.3. Characterization techniques Wide angle X-Ray diffraction spectra (WAXS) were recorded using a Thermo ARL diffractometer X-tra 48, at room temperature in the range of 2–30° (2θ) (step size = 0.02° (2θ), and scanning rate = 2 s/step) by using filtered Cu Kα radiation (λ = 1.54 ). Field Emission Scanning Electron Microscopy (FESEM) was carried out on the cryogenic fracture surfaces of the specimens using a Supra 25 Zeiss microscope. Before analysis, the surface was coated by sputtering with gold. Differential Scanning Calorimetry (DSC) tests were carried out on a Mettler Toledo DSC-822 under nitrogen atmosphere, sample size 5–6 mg in hermetically closed aluminium pans. Thermal history of samples was erased by a preliminary heating cycle at 10 °C/min from 0 °C to 250 °C, followed by cooling at 10 °C/min. Crystallization temperature (Tc) and enthalpy (ΔHc) were measured during the cooling run. The
a
2. Experimental 2.1. Materials The poly(lactic acid) (PLA) was a commercial grade (3000D) supplied by NatureWorks (USA). The fillers used were one organically modified montmorillonite supplied by Southern Clay (USA) -CLOISITE 30B-, one organically modified fluoro‐hectorite supplied by CO-OP CHEMICAL CO., LTD (Japan) -SOMASIF™ MEE- and one unmodified sepiolite supplied by Tolsa S.A (Spain) -PANGEL S9-. The characteristics of the clays used are listed in Table 1. 2.2. Preparation of nanocomposites 2.2.1. Melt mixing process Prior to the mixing step, PLA was dried at 50 °C under vacuum for 8 h and the clays at 90 °C under vacuum for 6 h. Nanocomposites were obtained at 5% and 7% of clay loading by melt blending using an internal
b
Table 1 Characteristics of clays. Type of clay
Commercial name
Modifier structure
Montmorillonitea
CLOISITE 30B
CLO30B
Synthetic fluorohectoritea
SOMASIF MEE
SOM MEE
Sepiolite
PANGEL S9
NA
Notation in text
SEPS9
HT = linear alkyl chains, R: C8~18 from hydrogenated tallow. a Organic modifier content ca. 30 wt% according to the technical data sheet.
Fig. 1. WAXS patterns of pristine clays and nanocomposites based on CLO30B (a) and SOM MEE (b).
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Fig. 2. FESEM micrographs of neat PLA (a), PLA with 5% CLO30B (b) and PLA with 5% SOM MEE (c).
glass transition (Tg), cold crystallization (Tcc) and melting (Tm) temperatures, as well as the cold crystallization (ΔHcc) melting (ΔHm) enthalpies were determined from second heating scans at 10 °C/min. Melting and crystallization temperatures were measured at maximum rate of the endothermal (Tm) or exothermal (Tcc, Tc) process respectively. The room temperature crystallinity (χ) of the materials was evaluated taking into account the content of clay present in nanocomposites using the following expression: 0
1 ΔH m −ΔHcc @ A 100 χ¼ %wt ΔHm o 1− 100clay
ð1Þ
where ΔHm is the specific melting enthalpy of the sample, ΔHcc is the speo cific cold crystallization enthalpy of the sample, ΔHm is the specific melting enthalpy of the 100% crystalline PLA (93.0 J/g [32,33]) and %wtclay is the weight percentage of clay. Thermogravimetric analysis (TGA) was carried out on 10 mg samples heated at 10 °C/min from 50 °C to 800 °C under air or nitrogen flow (60 cc/min) using a TA TGA Q500 thermobalance. The degradation temperatures taken into account were those at onset of weight loss (5%, T5%) and of maximum weight loss rate (Tmax). Dynamic Mechanical Thermal Analysis (DMTA) was performed on compression moulded 6 × 20 mm 2 films, using a DMA TA Q800 in tension film clamp, in the temperature range from 30 to 80 °C, at a heating rate of 2 °C/min, 1 Hz frequency, preload of 0.01 N, in strain controlled mode and 15 micron of amplitude. All tests were made according to the Standard UNI EN ISO 6721. 3. Results and discussions 3.1. Morphology 3.1.1. Wide Angle X-Ray diffraction (WAXS) The WAXS diffraction patterns of PLA and nanocomposites are characterized by a broad peak with maximum approximately at 2θ = 17° (not shown here), indicating an amorphous PLA structure. The most significant features of composites are encountered in the lower angle range, which gives indications on the clay dispersion.
CLO30B presents a single diffraction peak at 2θ = 5.0° corresponding to the basal reflection (001) and accounts for 1.8 nm interlayer distance [34] (Fig. 1a), whereas SOM MEE is characterized by a major diffraction peak at 2θ = 4.3° corresponding to an interlayer distance of 2.1 nm [35,36] (Fig. 1b). The two organoclay structures also differ as far as the structural order is concerned, indeed the broad CLO30B (001) reflection peak highlights a large layer disorder whereas a much higher structural order can be evidenced for SOM MEE through its sharp basal reflection peak at 4.3°. CLO30B and SOM MEE appear to have a good interaction with PLA; indeed a shift of the main diffraction peak to lower angles is observed (Fig. 1a and b), corresponding to an increase of the interlayer distance around ca. 2.0 nm and 1.0 nm for CLO30B and SOM MEE, respectively. It is worth noticing that, despite both cases we can describe the nanocomposite as “intercalated”, their structure is quite different. As far as CLO30B is concerned, the formation of a disordered intercalated structure can be observed, evidenced through the broad clay (001) reflection which is directly related to the pristine clay disordered structure. On the other hand, the ordered structure of SOM MEE is even increased with the formation of sharper base peak in the nanocomposites as well as an increased intensity of the second order diffraction peak at 2θ =5.6. The dispersion of needle-shaped sepiolite, being this a non swelling clay, cannot be accessed through X-Ray diffraction. Indeed, only a dilution effect can be observed in the XRD spectra, not presented here, upon addition to PLA.
3.1.2. Field Emission Scanning Electron Microscopy (FESEM) Field Emission Scanning Electron Microscopy shows a good level of CLO30B (Fig. 2b) and SOM MEE (Fig. 2c) dispersion at 5% clay loading, as single dispersed clay platelets (see arrows in Fig. 2) and very few aggregates less than 1 μm size could be observed in the sample, in agreement with the high dispersion determined by means of WAXS analysis. Similar results were obtained at 7 wt% loading. In the case of SEPS9 based nanocomposites (Fig. 3), PLA+ 5% SEPS9 shows a good dispersion level of sepiolite particles (Fig. 3a and b), since a high number of single needles (see arrow in Fig. 3b) can be observed with the low occurrence of small clay needle bundles (ca. 1–2 microns,
Fig. 3. FESEM micrographs of PLA with 5% SEPS9: clay bundles (a) and single clay needles (b).
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Fig. 4. FESEM micrographs of PLA + 7% SEPS9.
arrow in Fig. 3a). In the case of PLA+ 7% SEPS9, single dispersed needles could be also observed in the sample fracture, however, several microaggregates of 4–6 microns were detected (see Fig. 4), indicating a reduced clay dispersion as compared to 5% loading. In general, the good level of dispersion of sepiolites in PLA can be attributed to high interactions between the polymer and SEPS9 originated from hydrogen bonding between the carbonyl groups of PLA and the hydroxyl groups rich edges of sepiolite which makes unnecessary organic modification differently from lamellar clays. 3.2. Thermal analysis (DSC) From the data reported in Table 2 it can be seen that the nanofillers have a quite strong effect on the crystallinity of the as moulded samples. The most influenced parameter is the crystallinity percentage: at low filler content a reduced crystallinity is observed with respect to neat PLA; at high filler contents this tendency is inverted and a crystallinity similar to that of pristine PLA is registered. Sepiolite represents an exception to this trend probably due to the poor dispersion of the 7% loading evidenced by FESEM analysis, as the presence of several sepiolite micro-aggregates could hinder segmental rearrangements of PLA chains during crystallization and restrict the formation of PLA crystals in the polymer matrix. On the other hand, no relevant influence on glass transition temperature (Tg) of PLA is brought by the presence of the clays. A different scenario is depicted when analysing crystallization and second heating scans. From the cooling scan data reported in Table 3 it is seen that, despite unvaried crystallization temperatures (Tc), ΔHc increases for nanocomposites, in particular at higher clay contents, indicating the promotion of the crystallization process upon addition of the nanoparticles. From the DSC second heating scans reported in Fig. 5 it can be seen that PLA crystallization on heating is strongly enhanced by the nanoparticles, as observed in crystallization on cooling. Indeed, a trend is seen with lamellar clays towards the decrease of temperature (Tcc) and increase of enthalpy (ΔHcc) of cold crystallization, particularly evident at higher nanofiller concentration (Table 3). In the case of sepiolite Table 2 DSC data on PLA and nanocomposites obtained by first heating.
3.3. Dynamic Mechanical Thermal Analysis (DMTA) Fig. 6 compares the temperature dependence of E′ of neat PLA and nanocomposites at 5% loading, showing considerable increase of E′ with the addition of all clays (Table 4), which becomes more noticeable with increasing temperature . In particular, the enhancement of E′ below Tg is around 50% for all materials (see Fig. 6, Table 4), and this is associated to the reinforcement effect of nanoclays as crystallinity of the as moulded samples is even reduced with respect to neat PLA (Table 3). This phenomenon is even more evident at high temperatures, fact that can be attributed to a restricted chain motion above Tg due to the presence of the nanoparticles [37], leading to an increase of E′ of 260%, 470% and 380% for PLA/ CLO30B, PLA/SOM MEE and PLA/SEPS9 respectively at 80 °C (Fig. 6, Table 4). Addition of 7% of layered silicates results in further increases of E′ below Tg of about 70% for PLA/CLO30B and 110% for PLA/SOM MEE (Table 4) which is again more evident at higher temperatures with E′ enhancements of 260% and 480% for CLO30B and SOM MEE, respectively. In the case of PLA+ 7% SEPS9, considerable increases of E′ were also observed below and above Tg (by ca. 30% and 200% at 30 °C and 80 °C, respectively – see Table 4), however, contrary to what has been observed for lamellar silicates based nanocomposites, these increases Table 3 DSC data on PLA and nanocomposites obtained by cooling and second heating scans. Cooling
Tcca (°C)
ΔHcc (J/g)
Tma (°C)
ΔHm (J/g)
χ (%)
Tg (°C)
PLA PLA + 5% CLO30B PLA + 7% CLO30B PLA + 5% SOM MEE PLA + 7% SOM MEE PLA + 5% SEPS9 PLA + 7% SEPS9
96 98 95 94 91 102 99
33 25 30 33 35 45 32
164 165 164 163 163 169 166
44 29 39 41 46 49 37
12 5 10 9 13 5 6
60 58 58 57 56 63 61
Temperatures quoted are peak temperatures.
containing nanocomposites, also an increase of melting temperature (Tm) is observed which indicates that sepiolite not only promotes crystallization of PLA but also induces a higher order of the crystalline zones. The largest influence on PLA crystallinity (χ) is obtained by adding 7% of CLO30B which increases this value from 10 to 30%, as determined from the second heating scan (Table 3). Also from second heating, no relevant influence on Tg of PLA could be observed.
Sample
Sample
a
Fig. 5. DSC thermograms of PLA and nanocomposites, second heating.
PLA PLA + 5% PLA + 7% PLA + 5% PLA + 7% PLA + 5% PLA + 7% a
CLO30B CLO30B SOM MEE SOM MEE SEPS9 SEPS9
Second heating
Tca (°C)
ΔHc (J/g)
Tcca (°C)
ΔHcc (J/g)
Tma (°C)
ΔHm (J/g)
χ (%)
Tg (°C)
93 94 93 93 91 92 94
3 3 7 4 6 5 9
100 98 96 95 92 102 100
20 25 21 31 35 33 29
163 164 163 163 161 167 165
29 39 46 44 51 46 47
10 21 30 15 19 15 21
58 58 58 58 53 62 62
Temperatures quoted are peak temperatures.
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modifiers and compatibilisers. We believe that this unmodified sepiolite can be regarded as a potentially interesting material for the enhancement of PLA thermal and thermo-mechanical properties, especially thanks to its ability to extend the field of existence of the polymer solid phase. 3.4. Thermogravimetric analysis (TGA)
Fig. 6. Temperature dependence of E′ and Tan Delta for PLA and nanocomposites at 5% clay loading.
were lower than those obtained upon addition of 5% sepiolite, probably associated to the lower clay dispersion level in PLA+ 7% SEPS9 as compared to PLA+ 5% SEPS9 in accordance with FESEM analysis. Despite the fact that the addition of the clays does not bring significant differences in the temperature of the Tan Delta maximum, an interesting feature is represented by the E′ values at pure PLA Tan Delta temperature (68 °C). In particular, the material containing sepiolite still retains at this temperature a high E′ value making this filler the most interesting one as far as the maintenance of high modulus in temperature is concerned. Another significant feature is the ability of all nanosystems taken in to consideration to extend the field of existence of the glassy state. Indeed, PLA losses 10% of its initial storage modulus value at 57 °C, whereas by loading 5% of clays this loss is shifted to 59 °C for CLO30B, 52 °C for SOM MEE and 65 °C for SEPS9. A similar trend could be observed for 7% loading. These results are even more interesting if compared to what observed by Li et al. [38] on systems specifically designed to increase PLA crystallinity by means of nucleating agents and special processing conditions. The authors could observe an increase in thermal resistance only when adding a determined amount of nucleating agents, plasticizer and using a high mould temperature to further favor the crystallization process. The systems taken into consideration in this paper represent an easier approach to overcome one of the most important PLA drawbacks. In conclusion, concerning lamellar silicates nanocomposites the highest improvements were reached at higher clay contents and for addition of SOM MEE, probably associated to its higher aspect ratio (ca. 5000–6000 [35]) as compared to that reported for CLO30B (ca. 280 [35]), able to create a higher contact area for polymer/filler interactions. It is important to underline the very high E′ enhancements reached after addition of 5% and 7% wt of sepiolite without the need of organic
Different mechanisms are proposed to describe PLA thermal degradation. McNeill et al. [39] propose a backbiting ester interchange reactions involving −OH chain ends, producing lactide, olygomers or acetaldehyde and carbon monoxide. Kopinke et al. [40] propose the formation of another cyclic transition intermediate leading to an olefinic double bond and a carboxyl group, the final product being in this case acrylic acid and oligomers with open chain structure possessing a double bond and a carboxyl as end groups. Fig. 7 reports TGA curves for PLA and nanocomposites in nitrogen and air. The thermal stability of the PLA matrix in nitrogen (Fig. 7a) is significantly improved by the presence of all clays (Table 5). Indeed an increase of both T5% and Tmax can be observed, especially after addition of SEPS9, independently of the clay content achieving similar values of T5% and Tmax upon incorporation of 5% and 7% clay (Table 5). In general, this behaviour is attributed to a barrier effect of the clay towards polymer decomposition products ablation, thus increasing both onset and maximum weight loss temperatures [37].
a
b
Table 4 E′ value of PLA and nanocomposites at different temperature ranges.
PLA PLA + 5% CLO30B PLA + 7% CLO30B PLA + 5% SOM MEE PLA + 7% SOM MEE PLA + 5% SEPS9 PLA + 7% SEPS9
E′ at 30 °C (MPa)
E′ at 68 °C (MPa)
E′’ at 80 °C (MPa)
Tan Delta (°C)
2700 3800 4532 4330 5731 4200 3530
35 74 383 76 275 1100 900
4 16 28 25 51 21 13
68 67 69 64 69 72 72
Fig. 7. TGA mass loss curves for neat PLA and its nanocomposites with 5 wt% clay loading in nitrogen (a) and air (b).
K. Fukushima et al. / Materials Science and Engineering C 32 (2012) 1790–1795
for DSC analysis as well as the Network of Excellence NANOFUNPOLY of the European Commission VI Programme Framework, for funding.
Table 5 TGA data on PLA and nanocomposites in nitrogen and air. In N2
PLA PLA + 5% CLO30B PLA + 7% CLO30B PLA + 5% SOM MEE PLA + 7% SOM MEE PLA + 5% SEPS9 PLA + 7% SEPS9
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In air
T5% (°C)
Tmax (°C)
T5% (°C)
Tmax (°C)
308 328 324 316 321 338 335
366 370 371 371 370 373 373
300 334 330 322 331 334 331
352 379 381 379 375 383 383
PLA matrix degradation is scarcely influenced by the presence of oxygen (Fig. 7b) with a decrease in Tmax of about 15 °C. The presence of nanofillers produces more remarkable effects than in nitrogen with increases in T5% and Tmax of 20–30 °C, resulting in a marked thermoxidative stability of the nanofilled systems which degrade at higher temperatures than in nitrogen (see Table 5). This behaviour was partially already reported in a previous work of ours [30] and may be attributed to the formation of char promoted by nanofillers in the presence of oxygen of air which, besides acting as efficient protection towards the action of oxygen, provides an effective thermal shielding. 4. Conclusions Nanocomposites of PLA with CLO30B, SOM MEE and SEPS9 at 5 and 7 wt% of clay loading prepared by melt blending, showed a good level of polymer/clay dispersion as well as considerable thermal and thermomechanical improvements in PLA, according to WAXS, FESEM, TGA and DMTA analysis. According to DSC, all clays, although reducing crystallinity of as moulded samples, were found to act as nucleating agents after DSC annealing and during controlled heating inducing a higher degree of crystallinity and promoting kinetics of crystallization, especially at higher clay contents. Concerning thermal properties, similar increases in the thermal stability of PLA were obtained for all clays independent of the filler content. The highest thermo-mechanical improvements in lamellar silicate nanocomposites seem to be more influenced by the aspect ratio of clay nanoplatelets, rather than polymer/clay compatibility, observing higher improvements upon addition of SOM MEE as compared to CLO30B. Sepiolites showed comparable thermo-mechanical improvements to some lamellar silicates highly dispersed in the polymer as well as a high ability to maintain high modulus values after glass transition temperature, due to its good dispersion and high compatibility with the PLA without the need of using modifiers or compatibilisers. Acknowledgments The authors would like to acknowledge Federica Meloni from Università di Perugia, Terni branch for FESEM micrographs, Prof. J.M. Kenny and Dr. S. Dottori from Università di Perugia – Terni branch
References [1] H. Tsuji, Polylactides, Biopolymers for Medical and Pharmaceutical Applications, Wiley-VCH, Germany, 2005, pp. 187–220, Book, Chapter 7. [2] H. Tsuji, T. Ishizaka, Macromol. Biosci. 1 (2001) 59–65. [3] E. Pollet, M.A. Paul, P. Dubois, Biodegradable Polymers and Plastics, Kluwer Academic/Plenum Publishers, USA, 2003, pp. 535–542, Book, Chapter 21. [4] A. Hoshino, M. Tsuji, M. Ito, M. Momochi, A. Mizutani, K. Takakuwa, et al., , chapter 21, in: E. Chiellini, R. Solaro (Eds.), Study of the aerobic biodegradability of plastics materials under controlled compost, Plenum Press, New York, 2003, p. 47. [5] L. Liu, S. Li, H. Garreau, M. Vert, Biomacromolecules 1 (2000) 350–359. [6] H. Tsuji, Y. Tezuka, K. Yamada, J. Polymer Sci., Part B: Polymer Phys. 43 (2005) 1064–1075. [7] S. Li, M. Tenon, H. Garreau, C. Braud, M. Vert, Polym. Degrad. Stab. 67 (2000) 85–90. [8] D.F. Williams, Eng. Med. 10 (1981) 5–7. [9] H. Pranamuda, A. Tsuchii, Y. Tokiwa, Macromol. Biosci. 1 (2001) 25–29. [10] A. Hoshino, Y. Isono, Biodegradation 13 (2002) 141–147. [11] E. Chiellini, R. Solaro, Biodegradable Polymers and Plastics, Kluwer Academic/Plenum Publishers, USA, 2003. [12] S. Sinha Ray, M. Bousmina, Prog. Mater. Sci. 50 (2005) 962–1079. [13] A.B. Morgan, J.W. Gilman, J. Appl. Polym. Sci. 87 (2003) 1329–1338. [14] M.A. Paul, M. Alexandre, P. Degee, C. Calberg, R. Jerome, P. Dubois, Macromol. Rapid Commun. 24 (2003) 561–566. [15] M.A. Paul, C. Delcourt, M. Alexandre, P. Degee, F. Monteverde, A. Rulmont, P. Dubois, Macromol. Chem. Phys. 206 (2005) 484–498. [16] H. Chen, M. Zheng, H. Sun, Q. Jia, Mater. Sci. Eng., A 445–446 (2007) 725–730. [17] J.L. Feijoo, L. Cabedo, E. Giménez, J.M. Lagaron, J.J. Saura, J. Mater. Sci. 40 (2005) 1785–1788. [18] S. Sinha Ray, K. Okamoto, K. Yamada, M. Okamoto, Nano Lett. 2 (2002) 423–426. [19] S. Sinha Ray, K. Yamada, M. Okamoto, K. Ueda, Nano Lett. 2 (2002) 1093–1096. [20] S. Sinha Ray, P. Maiti, M. Okamoto, K. Yamada, K. Ueda, Macromolecules 35 (2002) 3104–3110. [21] S. Sinha Ray, K. Yamada, A. Ogami, M. Okamoto, K. Ueda, Macromol. Rapid Commun. 23 (2002) 493–497. [22] S. Sinha Ray, K. Yamada, M. Okamoto, A. Ogami, K. Ueda, Chem. Mater. 15 (2003) 1456–1465. [23] F. Vilaplana, E. Strömberg, S. Karlsson, Polym. Degrad. Stab. 95 (2010) 2147–2161. [24] S.R. Lee, H.M. Park, H. Lim, T. Kang, X. Li, W.J. Cho, C.S. Ha, Polymer 43 (2002) 2495–2500. [25] M.A. Paul, C. Delcourt, M. Alexandre, P. Degée, F. Monteverde, P. Dubois, Polym. Degrad. Stab. 87 (2005) 535–542. [26] N.K. Singh, B.D. Purkayastha, J.K. Roy, R.M. Banik, M. Yashpal, G. Singh, S. Malik, P. Maiti, ACS Appl. Mater. Interfaces 2 (2009) 69–81. [27] K. Fukushima, C. Abbate, D. Tabuani, M. Gennari, G. Camino, Polym. Degrad. Stab. 94 (2009) 1646–1655. [28] M. Arena, C. Abbate, K. Fukushima, M. Gennari, Environ. Sci. Pollut. Res. 18 (2011) 865–870. [29] K. Fukushima, D. Tabuani, C. Abbate, M. Arena, L. Ferreri, Polym. Degrad. Stab. 95 (2010) 2049–2056. [30] K. Fukushima, D. Tabuani, G. Camino, Mater. Sci. Eng. C 29 (2009) 1433–1441. [31] J.L. Ahlrichs, C. Serna, J.M. Serratosa, Clays Clay Miner. 34 (1975) 119–124. [32] E. Fukada, Biorheology 32 (1995) 593. [33] J.F. Turner, A. Riga, A. O'Connor, J. Zhang, J. Collis, J. Therm. Anal. Calorim. 75 (2004) 257–268. [34] Southern clay products/a subsidiary of Rockwood specialties, Inc., Cloisite® 30B Typical Physical Properties Bulletin, www.scprod.com. [35] L.A. Utracki, M. Sepehr, E. Boccaleri, Polym. Adv. Technol. 18 (2007) 1–37. [36] D.H.S. Souza, K. Dahmouche, C.T. Andrade, M.L. Dias, Appl. Clay Sci. 54 (2011) 226–234. [37] S. Ray, M. Okamoto, Prog. Polym. Sci. 28 (2003) 1539–1641. [38] H. Li, M.A. Huneault, Polymer 48 (2007) 6855–6866. [39] I.C. Mc Neill, H.A. Leiper, Polym. Degrad. Stab. 11 (1985) 267–309. [40] F.D. Kopinke, M. Remmler, K. Mackenzie, M. Möder, O. Wachsen, Polym. Degrad. Stab. 53 (1996) 329–342.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Poly(lactic acid)/clay nanocomposites: effect of nature and content of clay on morphology, thermal and thermo-mechanical properties Kikku Fukushima a,⁎, Daniela Tabuani b, Giovanni Camino a a b
Politecnico di Torino sede di Alessandria, INSTM research unit, Viale Teresa Michel 5, 15100 Alessandria,Italy PROPLAST Consortium, Viale Teresa Michel 5, 15100 Alessandria, Italy
a r t i c l e
i n f o
Article history: Received 22 August 2011 Received in revised form 5 March 2012 Accepted 22 April 2012 Available online 30 April 2012 Keywords: PLA Nanocomposites Layered silicates Sepiolite Thermo-mechanical properties
a b s t r a c t Nanocomposites were prepared by melt blending Poly(lactic acid) with 5 and 7 wt% of an organically modified montmorillonite or an organically modified magnesium sodium fluoro-hectorite or unmodified sepiolite. All nanocomposites show a good level of clay dispersion into the polymer matrix as well as a considerable thermal and thermo-mechanical properties improvement. According to thermal analysis, the clays seem to act as nucleating agents inducing a higher degree of crystallinity of the polymer and rate of crystallization. Similar increases in the thermal stability of Poly(lactic acid) were obtained for all clays. Concerning layered silicate nanocomposites, it was found that the main influencing factors on the thermo-mechanical properties appear to be the aspect ratio and dispersion of clay nanoplatelets, rather than polymer/clay chemical compatibility. Needle like sepiolite shows thermo-mechanical improvements comparable to some layered silicates and an interesting ability to maintain high storage modulus values at increasing temperatures, due to its good dispersion within the polymer without the need of organic modifiers as instead necessary for layered clays used in this work. © 2012 Elsevier B.V. All rights reserved.
1. Introduction During the past few decades, poly(lactic acid) (PLA) has attracted much attention for ecological, biomedical and pharmaceutical applications because it can be produced from renewable resources such as starch, it has good mechanical properties, it is biodegradable in the human body as well as in natural environments and the toxicity of its degradation products (lactic acid and oligomers) is very low [1–3]. Even more, as far as the biodegradability of PLA is concerned, it has been reported that the resulting products of the PLA hydrolytic degradation can be totally assimilated by microorganisms such as fungi or bacteria [4–7]; for example a PLA-degrading microorganism, Amycolatopsis sp., was recently isolated and characterized [5]. Other reports have suggested that enzymes have a significant role in PLA degradation [8–10]. In this way, Hoshino and Isono [10] have reported that PLA can be completely degraded at 55 °C, pH 8.5 in 20 days by the commercial lipase PL, derived from Alcaligenes sp. They concluded that complete degradation of PLA resulted from two processes: Firstly, the chemical hydrolysis from PLA into oligomers at higher pH and/or under higher temperature conditions, because polyesters are generally not stable under such conditions. Secondly, the enzymatic hydrolysis from the oligomers to the monomer.
⁎ Corresponding author. E-mail address: [email protected] (K. Fukushima). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.04.047
The main limitations of PLA towards its industrial application are its poor thermal and mechanical resistance, which limit its access to industrial sectors in which its use would be justified when biodegradability is required [3,11]. The above drawbacks could be overcome by enhancing the thermo-mechanical properties of PLA through copolymerization, blending and filling techniques. Indeed, the addition of nano-sized fillers in biodegradable polymers has a strong potential for designing eco-friendly materials for several applications, as the efficiency of nanofillers can be significant thanks to their high aspect ratio [3,12–15]. Among the most used nanofillers, layered silicates have been the most studied and were found to improve, even at low filler content (1–5 wt%), the mechanical, thermal, barrier and flame retardance properties of polymers, in comparison to unfilled matrices and to conventional microcomposites [13,16,17]. As far as biodegradable polymers are concerned, this route has been explored by Ray et al. [18–21], who prepared a series of PLA with various types of organically modified layered silicates, achieving important improvements in thermal and mechanical properties. In a subsequent research, the same group [22] prepared PLA nanocomposites with organically modified synthetic fluoromica, finding that PLA chains were highly intercalated within the clay. In parallel, the design of new nano-biocomposites should contain an assessment of the ecotoxicity of the degradation products after fragmentation, to ensure the effective assimilation by microorganisms without any hazardous effects on them and the environment [23]. Until now, the knowledge about the environmental and toxicological
K. Fukushima et al. / Materials Science and Engineering C 32 (2012) 1790–1795
effects of these newly produced nanomaterials is limited; it is expected, though, that these implications will be completely specific to the type of base material, and its size and shape. Generally, the addition of nanoclays in polymer matrices has been widely reported in the literature to improve the biodegradation of the resulting nanocomposites [24,25], although the exact mechanisms are not well known yet. This behaviour has been generally attributed to the high relative hydrophilicity of the clays, allowing an easier permeability of water into the material, thus accelerating the hydrolytic degradation process of the polymer. Moreover, the different pHs that arise in the proximities of the nanoclays after water absorption may catalyse the enzymatic activity of depolymerase and contribute to a remarkable enhancement of the biodegradation rate in nanocomposites [23], as it has been demonstrated in biodegradation studies using enzymes, compost and fungi media [26]. Preliminary toxicological tests in compost and by an isolated bacterium from the same compost on PLA reinforced with the organically modified layered silicates used in this work [27–29], have indicated that the biodegradation of these materials seems to be ecologically safe, observing no mortality in the bacterial colonies after nanocomposite incubation. In a previous work of ours [30] we have evidenced the possibility of sepiolite to be effectively dispersed without the need of organic modifiers, due to its large quantity of silanol groups located on surface and edges of each single needle [31], which could significantly improve its compatibility with polymer matrix. The aim of this work is to evaluate the effect of three nanoparticles with different chemical and physical structure on the thermal and thermo-mechanical properties of PLA, which is one of the most promising biodegradable polymers for a variety of industrial applications.
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mixer (Rheomix-Brabender OHG 47055) with a mixing time of 5 min, at 165 °C. The mixing was performed at two different rotor speeds: 30 rpm in the loading step and 60 rpm during mixing. The batch was extracted from the mixing chamber manually, allowed to cool to room temperature in air and ground in a rotatory mill. Sheets were obtained by compression molding in a hot-plate hydraulic press at 210 °C followed by cooling at room temperature under pressure. Characterizations were made on 0.5 mm films. 2.3. Characterization techniques Wide angle X-Ray diffraction spectra (WAXS) were recorded using a Thermo ARL diffractometer X-tra 48, at room temperature in the range of 2–30° (2θ) (step size = 0.02° (2θ), and scanning rate = 2 s/step) by using filtered Cu Kα radiation (λ = 1.54 ). Field Emission Scanning Electron Microscopy (FESEM) was carried out on the cryogenic fracture surfaces of the specimens using a Supra 25 Zeiss microscope. Before analysis, the surface was coated by sputtering with gold. Differential Scanning Calorimetry (DSC) tests were carried out on a Mettler Toledo DSC-822 under nitrogen atmosphere, sample size 5–6 mg in hermetically closed aluminium pans. Thermal history of samples was erased by a preliminary heating cycle at 10 °C/min from 0 °C to 250 °C, followed by cooling at 10 °C/min. Crystallization temperature (Tc) and enthalpy (ΔHc) were measured during the cooling run. The
a
2. Experimental 2.1. Materials The poly(lactic acid) (PLA) was a commercial grade (3000D) supplied by NatureWorks (USA). The fillers used were one organically modified montmorillonite supplied by Southern Clay (USA) -CLOISITE 30B-, one organically modified fluoro‐hectorite supplied by CO-OP CHEMICAL CO., LTD (Japan) -SOMASIF™ MEE- and one unmodified sepiolite supplied by Tolsa S.A (Spain) -PANGEL S9-. The characteristics of the clays used are listed in Table 1. 2.2. Preparation of nanocomposites 2.2.1. Melt mixing process Prior to the mixing step, PLA was dried at 50 °C under vacuum for 8 h and the clays at 90 °C under vacuum for 6 h. Nanocomposites were obtained at 5% and 7% of clay loading by melt blending using an internal
b
Table 1 Characteristics of clays. Type of clay
Commercial name
Modifier structure
Montmorillonitea
CLOISITE 30B
CLO30B
Synthetic fluorohectoritea
SOMASIF MEE
SOM MEE
Sepiolite
PANGEL S9
NA
Notation in text
SEPS9
HT = linear alkyl chains, R: C8~18 from hydrogenated tallow. a Organic modifier content ca. 30 wt% according to the technical data sheet.
Fig. 1. WAXS patterns of pristine clays and nanocomposites based on CLO30B (a) and SOM MEE (b).
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Fig. 2. FESEM micrographs of neat PLA (a), PLA with 5% CLO30B (b) and PLA with 5% SOM MEE (c).
glass transition (Tg), cold crystallization (Tcc) and melting (Tm) temperatures, as well as the cold crystallization (ΔHcc) melting (ΔHm) enthalpies were determined from second heating scans at 10 °C/min. Melting and crystallization temperatures were measured at maximum rate of the endothermal (Tm) or exothermal (Tcc, Tc) process respectively. The room temperature crystallinity (χ) of the materials was evaluated taking into account the content of clay present in nanocomposites using the following expression: 0
1 ΔH m −ΔHcc @ A 100 χ¼ %wt ΔHm o 1− 100clay
ð1Þ
where ΔHm is the specific melting enthalpy of the sample, ΔHcc is the speo cific cold crystallization enthalpy of the sample, ΔHm is the specific melting enthalpy of the 100% crystalline PLA (93.0 J/g [32,33]) and %wtclay is the weight percentage of clay. Thermogravimetric analysis (TGA) was carried out on 10 mg samples heated at 10 °C/min from 50 °C to 800 °C under air or nitrogen flow (60 cc/min) using a TA TGA Q500 thermobalance. The degradation temperatures taken into account were those at onset of weight loss (5%, T5%) and of maximum weight loss rate (Tmax). Dynamic Mechanical Thermal Analysis (DMTA) was performed on compression moulded 6 × 20 mm 2 films, using a DMA TA Q800 in tension film clamp, in the temperature range from 30 to 80 °C, at a heating rate of 2 °C/min, 1 Hz frequency, preload of 0.01 N, in strain controlled mode and 15 micron of amplitude. All tests were made according to the Standard UNI EN ISO 6721. 3. Results and discussions 3.1. Morphology 3.1.1. Wide Angle X-Ray diffraction (WAXS) The WAXS diffraction patterns of PLA and nanocomposites are characterized by a broad peak with maximum approximately at 2θ = 17° (not shown here), indicating an amorphous PLA structure. The most significant features of composites are encountered in the lower angle range, which gives indications on the clay dispersion.
CLO30B presents a single diffraction peak at 2θ = 5.0° corresponding to the basal reflection (001) and accounts for 1.8 nm interlayer distance [34] (Fig. 1a), whereas SOM MEE is characterized by a major diffraction peak at 2θ = 4.3° corresponding to an interlayer distance of 2.1 nm [35,36] (Fig. 1b). The two organoclay structures also differ as far as the structural order is concerned, indeed the broad CLO30B (001) reflection peak highlights a large layer disorder whereas a much higher structural order can be evidenced for SOM MEE through its sharp basal reflection peak at 4.3°. CLO30B and SOM MEE appear to have a good interaction with PLA; indeed a shift of the main diffraction peak to lower angles is observed (Fig. 1a and b), corresponding to an increase of the interlayer distance around ca. 2.0 nm and 1.0 nm for CLO30B and SOM MEE, respectively. It is worth noticing that, despite both cases we can describe the nanocomposite as “intercalated”, their structure is quite different. As far as CLO30B is concerned, the formation of a disordered intercalated structure can be observed, evidenced through the broad clay (001) reflection which is directly related to the pristine clay disordered structure. On the other hand, the ordered structure of SOM MEE is even increased with the formation of sharper base peak in the nanocomposites as well as an increased intensity of the second order diffraction peak at 2θ =5.6. The dispersion of needle-shaped sepiolite, being this a non swelling clay, cannot be accessed through X-Ray diffraction. Indeed, only a dilution effect can be observed in the XRD spectra, not presented here, upon addition to PLA.
3.1.2. Field Emission Scanning Electron Microscopy (FESEM) Field Emission Scanning Electron Microscopy shows a good level of CLO30B (Fig. 2b) and SOM MEE (Fig. 2c) dispersion at 5% clay loading, as single dispersed clay platelets (see arrows in Fig. 2) and very few aggregates less than 1 μm size could be observed in the sample, in agreement with the high dispersion determined by means of WAXS analysis. Similar results were obtained at 7 wt% loading. In the case of SEPS9 based nanocomposites (Fig. 3), PLA+ 5% SEPS9 shows a good dispersion level of sepiolite particles (Fig. 3a and b), since a high number of single needles (see arrow in Fig. 3b) can be observed with the low occurrence of small clay needle bundles (ca. 1–2 microns,
Fig. 3. FESEM micrographs of PLA with 5% SEPS9: clay bundles (a) and single clay needles (b).
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Fig. 4. FESEM micrographs of PLA + 7% SEPS9.
arrow in Fig. 3a). In the case of PLA+ 7% SEPS9, single dispersed needles could be also observed in the sample fracture, however, several microaggregates of 4–6 microns were detected (see Fig. 4), indicating a reduced clay dispersion as compared to 5% loading. In general, the good level of dispersion of sepiolites in PLA can be attributed to high interactions between the polymer and SEPS9 originated from hydrogen bonding between the carbonyl groups of PLA and the hydroxyl groups rich edges of sepiolite which makes unnecessary organic modification differently from lamellar clays. 3.2. Thermal analysis (DSC) From the data reported in Table 2 it can be seen that the nanofillers have a quite strong effect on the crystallinity of the as moulded samples. The most influenced parameter is the crystallinity percentage: at low filler content a reduced crystallinity is observed with respect to neat PLA; at high filler contents this tendency is inverted and a crystallinity similar to that of pristine PLA is registered. Sepiolite represents an exception to this trend probably due to the poor dispersion of the 7% loading evidenced by FESEM analysis, as the presence of several sepiolite micro-aggregates could hinder segmental rearrangements of PLA chains during crystallization and restrict the formation of PLA crystals in the polymer matrix. On the other hand, no relevant influence on glass transition temperature (Tg) of PLA is brought by the presence of the clays. A different scenario is depicted when analysing crystallization and second heating scans. From the cooling scan data reported in Table 3 it is seen that, despite unvaried crystallization temperatures (Tc), ΔHc increases for nanocomposites, in particular at higher clay contents, indicating the promotion of the crystallization process upon addition of the nanoparticles. From the DSC second heating scans reported in Fig. 5 it can be seen that PLA crystallization on heating is strongly enhanced by the nanoparticles, as observed in crystallization on cooling. Indeed, a trend is seen with lamellar clays towards the decrease of temperature (Tcc) and increase of enthalpy (ΔHcc) of cold crystallization, particularly evident at higher nanofiller concentration (Table 3). In the case of sepiolite Table 2 DSC data on PLA and nanocomposites obtained by first heating.
3.3. Dynamic Mechanical Thermal Analysis (DMTA) Fig. 6 compares the temperature dependence of E′ of neat PLA and nanocomposites at 5% loading, showing considerable increase of E′ with the addition of all clays (Table 4), which becomes more noticeable with increasing temperature . In particular, the enhancement of E′ below Tg is around 50% for all materials (see Fig. 6, Table 4), and this is associated to the reinforcement effect of nanoclays as crystallinity of the as moulded samples is even reduced with respect to neat PLA (Table 3). This phenomenon is even more evident at high temperatures, fact that can be attributed to a restricted chain motion above Tg due to the presence of the nanoparticles [37], leading to an increase of E′ of 260%, 470% and 380% for PLA/ CLO30B, PLA/SOM MEE and PLA/SEPS9 respectively at 80 °C (Fig. 6, Table 4). Addition of 7% of layered silicates results in further increases of E′ below Tg of about 70% for PLA/CLO30B and 110% for PLA/SOM MEE (Table 4) which is again more evident at higher temperatures with E′ enhancements of 260% and 480% for CLO30B and SOM MEE, respectively. In the case of PLA+ 7% SEPS9, considerable increases of E′ were also observed below and above Tg (by ca. 30% and 200% at 30 °C and 80 °C, respectively – see Table 4), however, contrary to what has been observed for lamellar silicates based nanocomposites, these increases Table 3 DSC data on PLA and nanocomposites obtained by cooling and second heating scans. Cooling
Tcca (°C)
ΔHcc (J/g)
Tma (°C)
ΔHm (J/g)
χ (%)
Tg (°C)
PLA PLA + 5% CLO30B PLA + 7% CLO30B PLA + 5% SOM MEE PLA + 7% SOM MEE PLA + 5% SEPS9 PLA + 7% SEPS9
96 98 95 94 91 102 99
33 25 30 33 35 45 32
164 165 164 163 163 169 166
44 29 39 41 46 49 37
12 5 10 9 13 5 6
60 58 58 57 56 63 61
Temperatures quoted are peak temperatures.
containing nanocomposites, also an increase of melting temperature (Tm) is observed which indicates that sepiolite not only promotes crystallization of PLA but also induces a higher order of the crystalline zones. The largest influence on PLA crystallinity (χ) is obtained by adding 7% of CLO30B which increases this value from 10 to 30%, as determined from the second heating scan (Table 3). Also from second heating, no relevant influence on Tg of PLA could be observed.
Sample
Sample
a
Fig. 5. DSC thermograms of PLA and nanocomposites, second heating.
PLA PLA + 5% PLA + 7% PLA + 5% PLA + 7% PLA + 5% PLA + 7% a
CLO30B CLO30B SOM MEE SOM MEE SEPS9 SEPS9
Second heating
Tca (°C)
ΔHc (J/g)
Tcca (°C)
ΔHcc (J/g)
Tma (°C)
ΔHm (J/g)
χ (%)
Tg (°C)
93 94 93 93 91 92 94
3 3 7 4 6 5 9
100 98 96 95 92 102 100
20 25 21 31 35 33 29
163 164 163 163 161 167 165
29 39 46 44 51 46 47
10 21 30 15 19 15 21
58 58 58 58 53 62 62
Temperatures quoted are peak temperatures.
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modifiers and compatibilisers. We believe that this unmodified sepiolite can be regarded as a potentially interesting material for the enhancement of PLA thermal and thermo-mechanical properties, especially thanks to its ability to extend the field of existence of the polymer solid phase. 3.4. Thermogravimetric analysis (TGA)
Fig. 6. Temperature dependence of E′ and Tan Delta for PLA and nanocomposites at 5% clay loading.
were lower than those obtained upon addition of 5% sepiolite, probably associated to the lower clay dispersion level in PLA+ 7% SEPS9 as compared to PLA+ 5% SEPS9 in accordance with FESEM analysis. Despite the fact that the addition of the clays does not bring significant differences in the temperature of the Tan Delta maximum, an interesting feature is represented by the E′ values at pure PLA Tan Delta temperature (68 °C). In particular, the material containing sepiolite still retains at this temperature a high E′ value making this filler the most interesting one as far as the maintenance of high modulus in temperature is concerned. Another significant feature is the ability of all nanosystems taken in to consideration to extend the field of existence of the glassy state. Indeed, PLA losses 10% of its initial storage modulus value at 57 °C, whereas by loading 5% of clays this loss is shifted to 59 °C for CLO30B, 52 °C for SOM MEE and 65 °C for SEPS9. A similar trend could be observed for 7% loading. These results are even more interesting if compared to what observed by Li et al. [38] on systems specifically designed to increase PLA crystallinity by means of nucleating agents and special processing conditions. The authors could observe an increase in thermal resistance only when adding a determined amount of nucleating agents, plasticizer and using a high mould temperature to further favor the crystallization process. The systems taken into consideration in this paper represent an easier approach to overcome one of the most important PLA drawbacks. In conclusion, concerning lamellar silicates nanocomposites the highest improvements were reached at higher clay contents and for addition of SOM MEE, probably associated to its higher aspect ratio (ca. 5000–6000 [35]) as compared to that reported for CLO30B (ca. 280 [35]), able to create a higher contact area for polymer/filler interactions. It is important to underline the very high E′ enhancements reached after addition of 5% and 7% wt of sepiolite without the need of organic
Different mechanisms are proposed to describe PLA thermal degradation. McNeill et al. [39] propose a backbiting ester interchange reactions involving −OH chain ends, producing lactide, olygomers or acetaldehyde and carbon monoxide. Kopinke et al. [40] propose the formation of another cyclic transition intermediate leading to an olefinic double bond and a carboxyl group, the final product being in this case acrylic acid and oligomers with open chain structure possessing a double bond and a carboxyl as end groups. Fig. 7 reports TGA curves for PLA and nanocomposites in nitrogen and air. The thermal stability of the PLA matrix in nitrogen (Fig. 7a) is significantly improved by the presence of all clays (Table 5). Indeed an increase of both T5% and Tmax can be observed, especially after addition of SEPS9, independently of the clay content achieving similar values of T5% and Tmax upon incorporation of 5% and 7% clay (Table 5). In general, this behaviour is attributed to a barrier effect of the clay towards polymer decomposition products ablation, thus increasing both onset and maximum weight loss temperatures [37].
a
b
Table 4 E′ value of PLA and nanocomposites at different temperature ranges.
PLA PLA + 5% CLO30B PLA + 7% CLO30B PLA + 5% SOM MEE PLA + 7% SOM MEE PLA + 5% SEPS9 PLA + 7% SEPS9
E′ at 30 °C (MPa)
E′ at 68 °C (MPa)
E′’ at 80 °C (MPa)
Tan Delta (°C)
2700 3800 4532 4330 5731 4200 3530
35 74 383 76 275 1100 900
4 16 28 25 51 21 13
68 67 69 64 69 72 72
Fig. 7. TGA mass loss curves for neat PLA and its nanocomposites with 5 wt% clay loading in nitrogen (a) and air (b).
K. Fukushima et al. / Materials Science and Engineering C 32 (2012) 1790–1795
for DSC analysis as well as the Network of Excellence NANOFUNPOLY of the European Commission VI Programme Framework, for funding.
Table 5 TGA data on PLA and nanocomposites in nitrogen and air. In N2
PLA PLA + 5% CLO30B PLA + 7% CLO30B PLA + 5% SOM MEE PLA + 7% SOM MEE PLA + 5% SEPS9 PLA + 7% SEPS9
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In air
T5% (°C)
Tmax (°C)
T5% (°C)
Tmax (°C)
308 328 324 316 321 338 335
366 370 371 371 370 373 373
300 334 330 322 331 334 331
352 379 381 379 375 383 383
PLA matrix degradation is scarcely influenced by the presence of oxygen (Fig. 7b) with a decrease in Tmax of about 15 °C. The presence of nanofillers produces more remarkable effects than in nitrogen with increases in T5% and Tmax of 20–30 °C, resulting in a marked thermoxidative stability of the nanofilled systems which degrade at higher temperatures than in nitrogen (see Table 5). This behaviour was partially already reported in a previous work of ours [30] and may be attributed to the formation of char promoted by nanofillers in the presence of oxygen of air which, besides acting as efficient protection towards the action of oxygen, provides an effective thermal shielding. 4. Conclusions Nanocomposites of PLA with CLO30B, SOM MEE and SEPS9 at 5 and 7 wt% of clay loading prepared by melt blending, showed a good level of polymer/clay dispersion as well as considerable thermal and thermomechanical improvements in PLA, according to WAXS, FESEM, TGA and DMTA analysis. According to DSC, all clays, although reducing crystallinity of as moulded samples, were found to act as nucleating agents after DSC annealing and during controlled heating inducing a higher degree of crystallinity and promoting kinetics of crystallization, especially at higher clay contents. Concerning thermal properties, similar increases in the thermal stability of PLA were obtained for all clays independent of the filler content. The highest thermo-mechanical improvements in lamellar silicate nanocomposites seem to be more influenced by the aspect ratio of clay nanoplatelets, rather than polymer/clay compatibility, observing higher improvements upon addition of SOM MEE as compared to CLO30B. Sepiolites showed comparable thermo-mechanical improvements to some lamellar silicates highly dispersed in the polymer as well as a high ability to maintain high modulus values after glass transition temperature, due to its good dispersion and high compatibility with the PLA without the need of using modifiers or compatibilisers. Acknowledgments The authors would like to acknowledge Federica Meloni from Università di Perugia, Terni branch for FESEM micrographs, Prof. J.M. Kenny and Dr. S. Dottori from Università di Perugia – Terni branch
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