Chapter 11
Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential Downloaded from pubs.acs.org by UNIV OF IOWA on 10/29/18. For personal use only.
Wet Feeding Approach for Cellulosic Materials/PCL Biocomposites Giada Lo Re*,1 and Valentina Sessini2 1Division
of Biocomposites, Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden 2Laboratory of Polymeric and Composite Materials, University of Mons – UMONS, Place du Parc 23, 7000 Mons, Belgium *E-mail:
[email protected].
In the past decades, cellulosic materials attracted increasing interest for their potential as reinforcement in bioplastics due to their intrinsic strength and light weight, although uniform fiber dispersion is a challenge. Among biodegradable polyesters, polycaprolactone (PCL) has regained attention for its biodegradability in marine environment together with its ductility. Its low strength, petroleum-based origin and comparably high cost, limit the use of PCL. PCL, therefore, is a good candidate for beneficial effect of cellulose material addition for the preparation of biodegradable composites with improved properties. A one-step wet compounding is reported in this chapter to validate a sustainable method to improve the cellulose dispersion in a hydrophobic polymer matrix as PCL. A comparison between cellulosic wood pulp fibers, microfibrillated cellulose and nanofibrils is made to assess the feasibility of the wet feeding approach for the processing of the biocomposites. Assessment of matrix molar mass demonstrated that PCL is insensitive to the presence of the water during the melt compounding. FE-SEM and X-ray tomography was used to characterize the morphology and to evaluate the nanofibrillation. Tensile tests were carried out to evaluate the mechanical properties of the biocomposites. The shortening and dispersion of the cellulose fibers after the melt © 2018 American Chemical Society
processing were evaluated. Young’s modulus values indicated that the wet feeding approach improve the dispersion of the cellulose and resulted in enhanced mechanical properties of the biocomposites. The beneficial effect of the wet feeding approach was greater for the pulp fibers compared to the microfibrillated cellulose or the nanofibrils due to a more efficient melt processing and more significant effect on the preservation of the fiber length and their aspect ratio.
Introduction The major global challenge about plastic waste accumulated in landfill or in nature, including the marine environment, is estimated that as much as 6.3 billion tons from the 8.3 billion tons of plastic produced (1). In this context, biodegradable polyesters emerged as potential matrices for the replacement of the traditional petroleum based resins. Both academic and industrial interest are called to improve their properties and strategies for a reduction in price that can make them competitive in the market (2). Among the polyesters produced at industrial scale, poly(ε-caprolactone) (PCL) recently is regaining interest because of its biodegradability in marine environment (3), together with advantages such as an easy melt processing and high strain to failure (4). However, the higher cost compared to polyolefins, its low Young’s Modulus (240-420 MPa) and tensile strength (16-24 MPa) limit the application field of PCL based materials (5). The relatively low cost of lignocellulosic materials, their availability from renewable resources and their high mechanical properties (i.e. the Young’s modulus of nanocrystalline cellulose is around 140-150 GPa (6, 7).) suggest their use as reinforcement for polymeric biocomposites (8–10), in particular for biodegradable composites (10–13). On the other hand, the high hydrophilicity and moisture sorption of the cellulosic materials result in their poor compatibility with hydrophobic polymers leading to poor filler dispersion in the resulting composites. Moreover, depending on the cellulosic source, lignocellulosic fillers are affected from low thermal stability because of their pyrolytic degradation above 140°C. At common processing temperatures for the prevalent thermoplastic matrices, i.e. for polypropylene (PP) and polylactide (PLA) (200°C), this cellulosic thermal degradation lead to an undesirable brownish discoloration and compromising the composites performance (14). The low melting point of PCL (c.a. 60°C) allows processing temperatures suitable for lignocellulosic materials so that thermal degradation may possibly be limited. Among the challenges of integration of cellulosic materials in thermoplastic polymer matrices due to compatibility issues, processing challenges related to drying are not trivial. The traditional drying prior to melt compounding of the cellulosic materials, regardless their initial morphology in the water dispersion, lead to substantial and irreversible hornification, i.e. irreversible fiber and fibril aggregation (15, 16) that makes difficult their re-dispersion in the polymer melt. Between melt compounding techniques, automotive industry uses a “wet feeding” strategy from water dispersion (17–19), for clay-based nylon or 210
polyolefin composites, allowing for the exfoliation of the clay and the production of high performance clay-based nanocomposites. Oksman et al. (20), prepared polylactide/microcrystalline cellulose composites with reduced agglomeration and increased mechanical properties by using a “liquid-feeding” strategy. In particular, microcrystalline cellulose was fed in the extruder from a solvent dispersion after treatment with N,N-dimethylacetamide (DMAc) containing lithium chloride (LiCl) as swelling/separation agent. Beaugrand and Berzin (21) demonstrated a water plasticization of sisal hemp fibers when melt processed humid (water content ranging from 10 to 22 wt%) resulting in a higher fiber L/D ratio by favoring interfiber decohesion and consequently improved mechanical properties for fibers/PCL biocomposites. Yano et al. (22) developed a new processing method that enables continuous microfibrillation of pulp and its melt compounding with powdered polypropylene (PP) by using a water slurry containing never-dried kraft pulp. For the resulting 50 wt%, microfibrillated cellulose/PP composite the Young’s modulus was two times higher than that of neat PP, and the tensile strength was 1.5 times higher. The present study contributes to the validation of a one-step wet feeding approach as effective green and sustainable method for the melt compounding of different cellulosic materials as reinforcement for PCL biocomposites. The presented approach leads to the production of different cellulosic materials/PCL biocomposites avoiding the agglomeration due to the predrying of the cellulosic materials or the use of organic solvent as well as solvent exchange steps prior to melt processing. The results demonstrate an effective and improved dispersion of the overall cellulosic materials used as reinforcement for the PCL biocomposites. However, the never-dry pulp resulted the more suitable for an efficient wet feeding, due to the lower amount of water, which evaporate during the melt compounding. This fact allows the preserving of the fiber length and high L/D ratio, which significantly affect the mechanical properties of the final biocomposites. The main results pave a route for a new generation of sustainable cellulosebased biocomposites produced by wet feeding approach during melt processing.
Experimental and Materials Materials PCL grade Capa™ 6506 (Mw 50000 g/mol, melt flow index = 11.3-5.2 g/10 min with 2.16 kg, 1” PVC die at 160°C) was kindly provided by Perstorp Holding AB, Sweden. Bleached softwood Kraft fibers from pine (K46) were supplied by SCA Forest Products (Östrand pulp mill, Timrå, Sweden). The pine pulp fibers were soaked in deionized water overnight and then dispersed with a laboratory reslusher operated at 30000 rpm (Ultra Turrex T 25 D IKA, Germany). They were afterwards washed with acetate buffer (pH 4.6) containing 2.4 NaClO2 for one hour at 60 °C, in order to remove any impurities, and then they were directly filtered to the final 22 % dry content in cellulosic fibers (PULP) in the material used for the melt processing. The base aqueous microfibrilar cellulose (MFC) gel, with an 211
MFC content of 2,4 and 10 wt%, was kindly supplied by Borregaard Industries Ltd., Norway. Enzymatic cellulose nanofibrils (CNF) were prepared as described below. The different morphologies of the starting cellulosic materials are shown in Figure 1. All reagents and solvents (VWR Prolabo) were used as received without further purification.
Figure 1. Pristine morphologies of the cellulosic materials used in this study. In particular: a) bleached pulp (bar 500 microns), b) microfibrillated cellulose (bar 100 microns) and c) enzymatic nanofibrils (bar 1 micron). Preparation of Enzymatic CNF CNF was prepared by enzymatic pretreatment of never-dried pulp (supplied by Nordic Paper, Sweden) containing 13.8 wt% hemicelluloses and 0.7 wt% lignin according to Henriksson et al. (23) Briefly, the process included enzymatic (Novozym 476) pretreatment of the pulp followed by eight passes through a microfluidizer (Microfluidics Inc., USA). After disintegration, CNF was obtained as a viscous gel with ~1.6 wt% dry content. The morphology of CNF was assessed by TEM analysis and the diameter distribution of the fibrils had an average of 11.6 nm with a standard deviation of 4.1 nm, based on 100 measurements. Fabrication of Cellulosic Material/PCL Biocomposites Prior to extrusion, PCL in powder form and varying the amount (from 0 to 20 in dry content) of PULP (22 wt% in dry content) or MFC (2.4 and 10 wt% in dry content) or CNF (1.5 wt% in dry content) were manually premixed prior to the “wet feeding” approach. Reference samples from the conventional “dry feeding” were prepared for the PULP, MFC and CNF/PCL biocomposite by drying each material for 8 h at 30°C in a vacuum oven, prior to extrusion. As shown in Figure 2, cellulosic materials were dispersed in the PCL matrix by melt-blending using a DSM co-rotating twin-screw micro-compounder (DSM, Holland, Explore, 5 cc) working at 120°C first at 30 rpm for 5 min for the feeding and then at 100 rpm for about 15 min. This was assumed to represent complete evaporation of water during the processing, also due to the achievement of a constant value of torque, recorded during the processing. After compounding, and according to the ISO 527-2 standard, dumbbell (1BA) for the tensile tests, were prepared by injection molding 212
using a HAAKE™ MiniJet-Pro (Thermo Fisher Scientific) with an injection pressure of 1000 bar, an oven temperature of 120°C and mold temperature set at 40°C. The composition of different material melt processed, their initial water content and acronym are reported in Table 1.
Figure 2. Schematic representation of the wet feeding approach for the preparation of the cellulosic material/PCL biocomposites.
Table 1. Acronyms, final composition and initial water content of the biocomposites prepared by using a micro-compounder at 120°C for 15 min acronym
PCL (wt%)
Cellulosic material (wt%)
Initial water (wt%)
PCL
100
0
0-50
3%CNF/PCL WF
97
3
64.9
3%CNF/PCL DF
97
3
0
5%CNF/PCL WF
95
5
75.5
5%CNF/PCL DF
95
5
0
3%MFC*/PCL
WF
97
3
56.3
5%MFC*/PCL
WF
95
5
67.0
5%MFC*/PCL
DF
95
5
0
10%MFC**/PCL
WF
90
10
47.0
20%MFC**/PCL
WF
80
20
64.3
20%MFC**/PCL
DF
80
20
0 Continued on next page.
213
Table 1. (Continued). Acronyms, final composition and initial water content of the biocomposites prepared by using a micro-compounder at 120°C for 15 min acronym
PCL (wt%)
Cellulosic material (wt%)
Initial water (wt%)
3%PULP/PCL WF
97
3
9.6
5%PULP/PCL WF
95
5
15.1
5%PULP/PCL DF
95
5
0
10%PULP/PCL WF
90
10
26.1
20%PULP/PCL WF
80
20
41.5
20%PULP/PCL DF
80
20
0
*
From the gel with an MFC content of 2.4 wt%. 10 wt%.
**
From the gel with an MFC content of
Characterization of Biocomposites Tensile tests of the resulting PCL-based biocomposites were performed on samples conditioned for 100 h at 23°C and 50% of relative humidity (RH). The tensile tests were carried out using a Single Column Table Top Instron 5944 tensile micro tester 5944 with a load force of 2 kN and a deformation rate of 100% (30 mm/min). Seven replicates were performed for each biocomposite formulation and data scattering was in the range of the 5-9%. Thermogravimetrical analysis (TGA) were obtained using a Mettler Toledo instrument, calibrated with Indium, under air and nitrogen flow, from ambient temperature to 800°C at a heating rate of 10°C/min. Size exclusion chromatography (SEC) was performed in CHCl3 (Fischer Scientific, HPLC grade) used as the eluent at a flow rate of 1.0 mL/min, and the injection volume was 50 mL. The apparatus consisted of a Waters 717 Plus autosampler and a Waters (model 510) solvent pump equipped with a PL-ELS 1000 light scattering detector and three PL gel 10 mm mixed B columns (3007.5 mm) from Polymer Laboratories. Narrow molar mass polystyrene standards were used for calibration. The data were processed with Millennium software version 3.20. The morphology of the fibers and biocomposites was analyzed by Scanning Electron Microscopy (SEM) and X-ray microtomography. High resolution scanning electron micrographs were taken by using a Hitachi SEM S-4800 (Japan) with an accelerating voltage of 1 kV. Finally, Pt/Pd (60/40) sputtered for 20 s at a current of 80 μA using a Cressington 208HR sputter coater prior to imaging. The specimens were conditioned under controlled nitrogen atmosphere at -100°C, then cryo-microsectioned using RMC MT-XL ultramicrotome, and finally Pt/Pd (60/40) sputtered for 20 s at a current of 80 μA using a Cressington 208HR sputter coater prior to imaging. Transmission electron microscopy (TEM) imaging were performed using a Hitachi HT7700 TEM at 100 kV accelerating voltage. CNF aqueous dispersions 214
were deposited onto hollow carbon-coated 400 mesh copper grids (TED PELLA, USA) and examined in the microscope after drying. X-ray microtomography was carried out on samples cut out from the injected dumbbell specimens by using an Xradia MicroXCT-200. Scanning conditions for the X-ray source were voltage 45 kV, power 4 W, current 88 µA. Number of projections was 1105, exposition time 15 s/projection. A distance from the detector and X-ray source was 7.5 mm and 30 mm, respectively. Used magnification was 20xs and pixel resolution 1.0865 µm while the analyzed volume was of 1 mm diameter and 1 mm length. Mean fiber length and fiber length distribution of original fibers as well as of extracted fibers from biocomposites were evaluated by means of a MorFi Compact fiber analyzer from TECHPAP (France). For this purpose, a dilute suspension of fibers (25 mg/L) was analyzed using an optics & flow cell measurement. Data of more than 3000 fibers were treated and the mean fiber length, mean weighted length, and the fiber length distribution were obtained. For the analysis of reinforcing fibers, biocomposites at 20 wt% fiber content were first submitted to Soxhlet extraction in chloroform for 8 hrs.
Results and Discussion Wet feeding presents different limitations, between them the water amount that can be handled to allow an effective compounding of the components and the simultaneous water evaporation during the processing (at temperature higher than the boiling point of water). For these reasons, due to the different dry content of cellulosic materials used in this study, an initial comparison was made at lower filler content (from 0 to 5 wt% in dry content). Mechanical Properties of Biocomposites In Figure 3.a and 3.b the Young’s modulus and the yield stress as a function of the filler content of neat PCL and its biocomposites (3-5 wt% in dry content) are reported, respectively. The wet feeding approach improved the overall mechanical properties of the biocomposites compared with dry feeding, regardless the starting different morphology of the cellulosic material. It is worth noting that the different processing approaches did not leaded to significant changes in the crystallinity of the polycaprolactone (lower than the 5%). The improvement in the mechanical properties due to the wet feeding can not be ascribed to changes in the polymer crystallinity (24). However, the initial aspect ratio of the cellulose seemed to play a key role when the component where fed from water dispersion. Microfibrillated cellulose has the lower aspect ratio between the cellulosic materials studied. During its production process, the size of the fibers is reduced by mechanical beating, which affect significantly the aspect ratio and the fiber length, generating hyperbranched structure partially micro end nanofibrillated. Indeed, CNF/PCL as well as PULP/ PCL biocomposites showed an improvement of the Young’s modulus and yield 215
stress, suggesting that a better dispersion of cellulosic materials into the PCL matrix was obtained compared with the biocomposites processed by dry feeding. For MFC/PCL biocomposites, a slightly increase of the Young’s modulus and the yield stress was observed only for the 3wt%MFC/PCL biocomposite. 5%MFC/ PCL biocomposite shown a decrease of the Young’s modulus while the yield stress remained almost constant.
Figure 3. Mechanical properties of different cellulosic materials/PCL biocomposites. a) Young’s modulus and b) Yield stress in function of the filler content (wt% in dry content).
216
To further investigate the effect of the wet feeding on higher filler content biocomposites, a comparison between the MFC/PCL and PULP/PCL biocomposites was studied. The amount of water for the system with CNF was higher than 75 wt% already for the biocomposites with filler content of 5 wt%. The wet feeding approach resulted therefore in a not efficient melt compounding and simultaneous evaporation of the water for CNF/PCL biocomposites with high amount of CNF, thus, they were not taken into account for the higher cellulose content biocomposites studied. DSC analysis, as reported elsewhere (24), confirmed a small decrease in the melting enthalpies in the wet fed materials compared to the dry fed one, suggesting that the wet feeding approach resulted in an inferior shear stress during the processing compared to the traditional dry feeding, in agreement with the torque values recorded during the processing. The mechanical properties of the MFC and PULP-based biocomposites are reported in Figure 4.a, 4.b and 4.c, were the Young’s modulus, the yield stress and the strain at break are reported, respectively. For MFC/PCL biocomposites, the wet feeding processing did not improve their mechanical properties. In fact, Young’s modulus and yield stress remained rather constant and the strain at break drastically decreased of about 2 order of magnitudes as the filler content increased. The reasons behind these results could conceivably be ascribed to the initial lower aspect ratio of the MFC compared to the pulp fiber, which resulted in a not significant effect of the wet feeding. On the contrary, the wet feeding approach resulted suitable for PULP/PCL biocomposites, leading to a great improvement of the Young’s modulus and the yield stress for increased filler content, while the strain at break decrease as for MFC/PCL biocomposites, as expected. In particular, Young’s modulus increased linearly from 0.25 GPa for net PCL to 1.85 GPa for 20%PULP/PCL WF while for its dry fed counterpart (20%PULP/PCL DF) a lower value was obtained, that is 0.59 GPa. A progressive increase was also observed for the yield stress values starting from 16 MPa for neat PCL to 31 MPa for 20%PULP/PCL WF. For comparison, Young’s modulus and yield stress values previously reported in literature are 0.71 GPa and 12 MPa for PCL reinforced with 20 wt% of medium length wood pulp cellulose fibers (25), respectively. Furthermore, for PCL filled with 20 wt% of borassus or ramie fibers, these values were reported (26) to be 0.6 and 1.1 GPa for the Young’s modulus, respectively, whereas 18 and 28 MPa for the yield stress, respectively. The results obtained in this work confirm that wet feeding approach improves mechanical properties of PULP-based biocomposites due to the better dispersion of the cellulosic fillers into the PCL matrix probably due to the preservation of the fiber length for the milder shear stress together with the avoiding of the initial agglomeration during the drying process prior to the compounding, as already observed by Beaugrand et Berzin (21). In order to find a confirmation of this proposed hypothesis for the improved reinforcement shown by the biocomposites prepared by the wet feeding, a more detailed study of the wet feeding effect on the thermal and morphological properties of the PULP-based biocomposites has been assessed by TGA, SEM 217
microscopy and X-ray microtomography. Moreover, the pulp fiber lengths of the fibers recovered after Soxhlet extraction from Chloroform were assessed.
Figure 4. Mechanical properties of different cellulosic materials/PCL biocomposites. a) Young’s Modulus, b) Yield stress and c) Strain at break as functions of the filler content (wt% in dry content). 218
Thermal, Structural, and Morphological Properties of PULP/PCL Biocomposites TGA analysis was performed to investigate a possible degradation of the matrix during the process, due to shear stress but also to the presence of water in the system during the melt compounding which can provoke the hydrolytic degradation of the polymer matrix (27). The main results of TGA analysis are summarized in Table 2. The initial degradation temperatures (T5wt%) were determined at 5 wt% mass loss while the maximum degradation temperatures (Td) were calculated from the peak of the first derivative of the TG curves (DTG).
Table 2. Main TGA analysis results for all the sample studied Sample
T5 wt% (°C)
Td (°C)
PCL
369
413
5%MFC/PCL WF
369
412
10%MFC/PCL WF
344
410
20%MFC/PCL WF
331
410
5%PULP/PCL WF
357
412
10%PULP/PCL WF
339
411
20%PULP/PCL WF
325
408
TGA results showed that increasing the cellulosic filler content into the PCL matrix, the thermal stability of the biocomposites slightly decreases compared with neat PCL. In particular, increasing the amount of fiber led to a corresponding decrease in T5wt% values of PULP/PCL biocomposites. The values recorded are 357°C, 339°C and 325°C for biocomposites reinforced with 5 wt%, 10 wt% and 20 wt% of PULP by wet feeding, respectively. In comparison, the T5wt% of neat PCL is 369°C, indicating slightly decreased thermal stability of the biocomposites. The same trend was observed also for the MFC/PCL biocomposites, for the sake of comparison. The differences between the T5wt% of the neat matrix compared to the T5wt% of the different biocomposites (ΔT5%) is reported in Figure 5 as a function of the PULP content. The obtained values indicated a linear relation between pulp fiber loading and ΔT5% attributed to the increased shear during processing of the biocomposite discussed below. Td values were 413°C for neat PCL and 412°C, 411°C, 408°C for biocomposites with 5 wt%, 10 wt% and 20 wt% pulp fibers by wet feeding, respectively. The differences in the thermal stability as well as in the initial degradation temperature were ascribed to the increasing of the shearing in the extruder during the melt processing. In fact, the screw force (directly proportional to the torque) was monitored and different sharing values were observed in the extruder, depending on the amount of water in the system during the processing of the different biocomposites compared to neat PCL (24). However, the presence 219
of water could hydrolyze ester bonds along the PCL backbone and contribute to the decrease of the biocomposites thermal stability (27). To discern this possible decrease of the PCL molar mass, neat PCL and biocomposites were compounded by varying the amount of water for the wet feeding (from 0 to 30 mL, i.e. from 0 to 50 wt%) and by changing the residential time in the micro-compounder (ranging from 5 to 30 minutes). After melt processing of the biocomposites, pulp fibers were collected from the insoluble fraction from the Soxhlet extraction in Chloroform, while the solubilized PCL was recovered by using a rotavap and then analyzed using SEC. In Table 3, the list of the different materials melt processed and their corresponding PCL molecular weights are reported. The peak value of the SEC curve (Mz+1) and polydispersity (Ð) of the PCL matrix were assessed before and after processing recovering the polymeric matrix from the soluble fraction after soxhlet extraction of the biocomposites (considering the main population). Overall, either the large excess of water (20% by volume corresponding to ≈50 wt%) and the processing time slightly affected on PCL Mw or dispersity index, being not changed by more than 7.5% change compared to neat PCL. These results confirmed the insensitivity of the PCL matrix to the hydrolyzation of the ester bonds along the PCL backbone in presence of the water during the melt compounding and suggested the use of the PCL as a suitable polymer matrix for the wet feeding approach.
Figure 5. Delta T5% between neat PCL and PCL/pulp fibers under nitrogen with a heating rate of 20°C/min. Comparing Mz+1 values, which were selected as internal comparative values not affected by possible integration errors, indeed differences are observed. After processing neat PCL, in the absence of water at 120°C, a 7.5% decrease in the Mz+1 was observed for neat PCL. In comparison, processing in the presence of 50% water only resulted in a 6.3% decrease of the corresponding peak value, conceivably due to the lower shear stress thanks to the presence of water. PCL 220
extracted from dry fed 20 wt% PULP/PCL biocomposites resulted in a 10% decrease in Mz+1. Whereas, biocomposites produced by wet feeding resulted in less than a 6% decrease in Mz+1. The decrease in molecular weight in particular from dry feeding indicates that increased shear stress resulted in decreased PCL Mz+1, consistent with the screw force values recorded during the processing (24).
Table 3. Main results of SEC analysis on the neat PCL matrix before and after melt processing and on the PCL recovered from the soluble fraction of the Soxhlet extraction of the biocomposites, after their melt processing Sample
Mw
*Mz+1
Ð
PCL 6506
159163
259412
1.5
PCL 30min
165678
270989
1.4
PCL-20H2O 5min
162039
258320
1.4
PCL-20H2O 10min
162151
258714
1.4
PCL-20H2O 15min
159058
268538
1.5
PCL-20H2O 20min
161703
263748
1.4
PCL-20H2O 30min
158788
256727
1.4
PCL from 5% PULP/PCL-WF
177600
272850
1.3
PCL from 10% PULP/PCL-WF
174180
267850
1.3
PCL from 20% PULP/PCL-WF
173660
263210
1.3
PCL from 20% PULP/PCL-DF
167093
249770
1.3
* This value are reported in order to avoid the possible integration errors and used to compare
the molecular weight of the different PCL fractions.
The morphology of PCL biocomposites reinforced with cellulosic fillers was investigated by SEM analysis. The SEM micrographs of the cryo-microsectioned surface of the biocomposite, reinforced with 20 wt% of pulp, produced by wet feeding as well as dry feeding approach, are reported in Figure 6. The cryo-microsectioned surface of the biocomposites processed by wet feeding appeared smoother and more uniform than the surface of dry fed biocomposites, which showed high roughness and porosity. Moreover, for dry fed biocomposites, fiber-fiber interactions were promoted by the drying process prior to melt compounding, leading to the agglomeration of the cellulosic filler and the formation of debonding at the interface between fibers and matrix. Regarding wet fed biocomposites, they showed better impregnation of the polymer matrix on the fibers and consequently improved interface between fibers and PCL. X-ray microtomographs of biocomposites produced using the wet feeding approach as well as the traditional dry feeding are reported in Figure 7 and the X-ray 3D reconstructions are reported in Figure 8. Significant changes in the morphology of the two different biocomposites are distinguished. Large extent of aggregation was observed for dry feeding 221
compared to wet feeding (Figures 7 and 8, right microtomographs for the biocomposites produced by dry feeding, left for the wet feeding). From the morphological analysis, it is evident that wet feeding favored the fiber dispersion and consequently the distribution within PCL matrix, preventing before and during processing the formation of strong fiber-fiber interactions resulting in agglomeration.
Figure 6. SEM micrographs at different magnifications of the biocomposites produced by wet feeding approach (micrographs on the left) or the traditional dry feeding (micrographs on the right). The 3D reconstruction of the X-ray tomography in Figure 8 highlighted the substantial effect of the wet feeding on in-situ nanofibrillation of the fibers during processing. The best results were therefore obtained for wet feeding, with well-distributed fibers and significant nanofibrillation for the 20%PULP/PCL biocomposites (Figures 7 and 8, left microtomographs). The in-situ nanofibrillation is mainly observed for wet feeding compared to the dry fed biocomposites, in which the fibers are clearly in large agglomerates and poorly distributed into the polymer matrix. Thus, morphological analysis of biocomposites demonstrated that wet feeding prevented the formation of agglomerates due to strong fiber-fiber interactions before and during processing, promoting the fiber dispersion and distribution within PCL matrix. Even better results were obtained in our recent published work for acetylated PULP/PCL biocomposites where the modified PULP-based biocomposites showed further improved dispersion of the fiber into the PCL matrix leading to improved mechanical properties, but only in the composites where the modified fiber were compounded with the PCL by using the wet feeding approach (24). 222
Figure 7. X-ray microtomographs of 20 wt% PULP-based biocomposites produced using the wet feding approach (micrograph on the left) or the traditional dry feding (micrograph on the right). Bars = 100 microns. The effect of the processing aproach on the mean fiber length and fiber length distribution was also investigated. The effect on the mechanical properties of the fiber length on fiber-based composites is renowned (21). Figure 9 shows the fiber length distributions of the fibers recovered from the Soxhlet extraction of the biocomposites. As expected, the mean fiber length in the biocomposites decreased with the increase of fiber content, underling once more the effect of the increased shear stress with the fiber content, in agreement with the TGA analysis. The fiber length decreased considerably after the extrusion process by wet feeding of 20 wt% PULP/PCL biocomposites, but much more for the fibers extracted from the traditional dry fed biocomposites. In particular, the mean fiber lengths for 223
pristine fibers of 1315 μm decreased to 465 μm for the fiber recovered from the 20%PULP/PCL biocomposites produced by wet feeding and to 352 μm for the dry fed ones. The overall fiber shortening was more remarkable for fiber compounded with PCL by dry feeding, reaching the lowest final mean fiber length. Wet feeding is therefore suggested to preserve the fiber length during melt processing, resulting in an increase in the aspect ratio of the fibers (24), which contribute to the improvement of the mechanical properties achieved.
Figure 8. Rendered 3D volume reconstruction of X-ray microtomographs of the biocomposites produced using the wet feeding approach (micrograph on the left) or the traditional dry feeding (micrograph on the right). The volume size is of 1mm diameter and 1mm length; false color.
Figure 9. Fiber length distribution of the fibers recovered after Soxhlet extraction from CHCl3 of the different PULP/PCL biocomposites prepared by wet feeding, and the 20 wt% biocomposites prepared by dry feeding. 224
Conclusion In the present chapter an easy and cost-effective method to produce high performance PCL based biocomposites is proposed and validated. The study highlights the importance of a wet feeding compounding approach compared to the traditional dry feeding one. The never-dry pulp resulted the more suitable for an efficient wet feeding, firstly due to the lower amount of water, which evaporates during the melt compounding. We demonstrated that wet feeding approach allows the preserving of the fiber length and high L/D ratio of the pulp fibers, which significantly affect the mechanical properties of the final biocomposites. Adding 20 wt% of PULP by wet feeding, mechanical properties were improved. In particular, the Young’s modulus increased linearly from 0.25 GPa for neat PCL to 1.85 GPa while for its dry fed counterpart a lower value was obtained, that is 0.59 GPa. A progressive increase was also observed for the yield stress values starting from 16 MPa for neat PCL to 31 MPa for 20%PULP/PCL WF. The results obtained in this work confirm that wet feeding approach improves mechanical properties of PULP-based biocomposites due to the better dispersion of the cellulosic filler into the PCL matrix, due to the preservation of the fiber length for the milder shear stress, together with the avoiding of the initial agglomeration during the drying prior to compounding. Therefore, cost-effective PULP/PCL-based biocomposites with outstanding mechanical performance are accomplished in the present study.
Acknowledgments Dr. G. Lo Re is grateful for funding provided by the Swedish agency SSF for the Grant No. GMT14-0036 on HIgh-performance CNF nanocomposites.
References 1. 2.
3.
4. 5.
6.
7.
Geyer, R.; Jambeck, J. R.; Law, K. L. Sci. Adv. 2017, 3, e1700782. Narayan, R., The Promise of Bioplastics–Bio-Based and BiodegradableCompostable Plastics. In Bio-Based Plastics: Materials and Applications; Wiley, 2014; pp 347−357. Kershaw, P. J., Biodegradable Plastics & Marine Litter: Misconceptions, Concerns and Impacts on Marine Environments; United Nations Environment Programme (UNEP): Nairobi KE, 2015. Woodruff, M. A.; Hutmacher, D. W. Prog. Polym. Sci. 2010, 35, 1217–1256. Lindblad, M. S.; Liu, Y.; Albertsson, A.-C.; Ranucci, E.; Karlsson, S., Polymers from renewable resources. In Degradable Aliphatic Polyesters; Springer: Berlin, Heidelberg, 2002; pp 139−161. Eichhorn, S. J.; Dufresne, A.; Aranguren, M.; Marcovich, N.; Capadona, J.; Rowan, S.; Weder, C.; Thielemans, W.; Roman, M.; Renneckar, S. J. Mater. Sci. 2010, 45 (1), 1. Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Chem. Soc. Rev. 2011, 40, 3941–3994. 225
8. 9. 10. 11.
12. 13. 14. 15. 16. 17. 18.
19. 20. 21. 22. 23. 24. 25. 26. 27.
Lo Re, G.; Morreale, M.; Scaffaro, R.; La Mantia, F. P. Polym. Int. 2012, 61, 1542–1548. Scaffaro, R.; Morreale, M.; Re, G. L.; La Mantia, F. J. Appl. Polym. Sci. 2009, 114, 2855–2863. Spinella, S.; Re, G. L.; Liu, B.; Dorgan, J.; Habibi, Y.; Leclere, P.; Raquez, J.M.; Dubois, P.; Gross, R. A. Polymer 2015, 65, 9–17. Oksman, K.; Aitomäki, Y.; Mathew, A. P.; Siqueira, G.; Zhou, Q.; Butylina, S.; Tanpichai, S.; Zhou, X.; Hooshmand, S. Composites, Part A 2016, 83, 2–18. Re, G. L.; Morreale, M.; Scaffaro, R.; La Mantia, F. P. J. Appl. Polym. Sci. 2013, 129, 3198–3208. Scaffaro, R.; Morreale, M.; Re, G. L.; La Mantia, F. Polym. Degrad. Stab. 2009, 94, 1220–1229. De la Orden, M.; Sánchez, C. G.; Quesada, M. G.; Urreaga, J. M. Polym. Degrad. Stab. 2010, 95, 201–206. Idström, A.; Brelid, H.; Nydén, M.; Nordstierna, L. Carbohydr. Polym. 2013, 92, 881–884. Newman, R. H. Cellulose 2004, 11, 45–52. Karger-Kocsis, J.; Kmetty, Á.; Lendvai, L.; Drakopoulos, S. X.; Bárány, T. Materials 2014, 8, 72–95. Rousseaux, D. D.; Sallem-Idrissi, N.; Baudouin, A.-C.; Devaux, J.; Godard, P.; Marchand-Brynaert, J.; Sclavons, M. Polymer 2011, 52, 443–451. Soulestin, J.; Quiévy, N.; Sclavons, M.; Devaux, J. Polym. Eng. Sci. 2007, 47, 467–476. Oksman, K.; Mathew, A. P.; Bondeson, D.; Kvien, I. Compos. Sci. Technol. 2006, 66, 2776–2784. Beaugrand, J.; Berzin, F. J. Appl. Polym. Sci. 2013, 128, 1227–1238. Suzuki, K.; Okumura, H.; Kitagawa, K.; Sato, S.; Nakagaito, A. N.; Yano, H. Cellulose 2013, 20, 201–210. Henriksson, M.; Henriksson, G.; Berglund, L.; Lindström, T. Eur. Polym. J. 2007, 43, 3434–3441. Lo Re, G.; Spinella, S.; Boujemaoui, A.; Vilaseca, F.; Larsson, P. T.; Adås, F.; Berglund, L. A. ACS Sustainable Chem. Eng. 2018, 6, 6753–6760. Cocca, M.; Avolio, R.; Gentile, G.; Di Pace, E.; Errico, M.; Avella, M. Carbohydr. Polym. 2015, 118, 170–182. Sarasini, F.; Tirillò, J.; Puglia, D.; Dominici, F.; Santulli, C.; Boimau, K.; Valente, T.; Torre, L. Compos. Struct. 2017, 167, 20–29. Seo, K. S.; Cloyd, J. D. J. Appl. Polym. Sci. 1991, 42, 845–850.
226
