Cellular biocomposites from polylactide and microfibrillated cellulose

Abstract

This paper describes the production of “green” microfibrillated cellulose-reinforced polylactide cellular biocomposites using a wet mixing technique combined with supercritical carbon dioxide foaming. The effect of composition on the morphology, density and compression modulus of foams was investigated for different processing parameters, along with the use of a chain extender to modify the melt elasticity. Foams with mean densities ranging from 0.18 to 0.32 g/cm³ were obtained for the neat polylactide and polylactide/5 wt% microfibrillated cellulose, respectively, and there was a corresponding increase in compressive modulus from 25 to 47 MPa. The addition of the chain extender is argued to compensate the molar mass loss induced by the different processing steps, promoting more uniform foam structures and allowing a density reduction of up to 75%.

Keywords

Foams biocomposites microfibrillated cellulose polylactide

Introduction

Ecological issues, increasing oil prices and dwindling natural resources are currently driving a major effort to find viable substitutes for petroleum-based materials. Part of this effort is devoted to the development of sustainable composite materials combining fully compostable matrices and fibers obtained from renewable resources. Polylactide (PLA), a linear aliphatic polyester first synthesized by Carothers in 1930,¹ is among the more versatile biodegradable polymers and can be derived from renewable sources of biomass. However, although PLA is generally considered to be a high strength, high modulus thermoplastic, its brittleness has limited its industrial implementation. In order to improve its mechanical performance without compromising its reputation as an environmentally friendly material, PLA has therefore been combined with a range of natural fiber reinforcements, including jute fiber, microcrystalline cellulose, wood flour, wood fiber²^–⁵ and, more recently, microfibrillated cellulose (MFC).⁶^–⁹ MFC is of particular interest in that it is not only renewable, biodegradable and recyclable, but also potentially capable of very high degrees of reinforcement at low-volume fractions owing to the relatively high aspect ratios of individual MFC fibers.^10,11

In fields of application such as the packaging and building industries, use of such biocomposites nevertheless remains limited owing to their low-impact resistance and high density.¹² Their transformation into foam-like structures would decrease their density, but may at the same time improve their specific mechanical properties, hence reducing materials costs. The associated processing routes should clearly involve neither emission of volatile organic compounds (VOCs) nor production of ozone depleting or greenhouse gases. The present work focuses on the development of PLA/MFC biocomposites considering a novel compounding technique and the use of supercritical carbon dioxide (CO₂) to produce PLA/MFC foams. CO₂ is a non-toxic, non-flammable, chemically inert, inexpensive and abundant gas, readily available directly from the atmosphere or as a by-product from industrial processes such as ammonia and ethanol production.¹³ Moreover, as physical foaming is used throughout, no additional CO₂ is directly generated during the foaming process.

A potential problem with the foaming of PLA is that its melt viscosity and melt elasticity may be limited owing to its typically low molar mass.^14,15 Moreover, like other polycondensates, PLA is sensitive to hygrothermal degradation during melt processing, which may lead to further decreases in molar mass.¹⁶ A chain extender was therefore used to increase the molecular weight by linking the reactive hydroxyl or carboxyl end-groups to form a branched architecture,^17,18 and hence improve the foamability of the PLA/MFC.¹⁹^–²¹

Experimental

Materials

A commercially available PLA foaming grade, 8052D from NatureWorks LLC (USA), was provided in the form of granules (GPLA). The granules were further ground by ICO Polymers (USA) into particles (PPLA) with diameters of between 15 and 150 µm. The glass transition temperature (T_g) and the melting temperature (T_m) were determined from differential scanning calorimetry (DSC, TA Instruments Q100) heating scans at 10 K/min to be 55°C and 156°C, respectively.

MFC isolated from bleached softwood pulp (Domsjö ECO Bright; Domsjö Fabriker AB, Sweden) was prepared at Innventia AB (Sweden) following a procedure described in detail elsewhere.²² The cellulose pulp was exposed to a combination of high pressure shear forces, enzymatic hydrolysis and a final homogenization step to give a water slurry with a dry MFC content of 3.1 wt%.

The chain extender Joncryl™ ADR-4368 from BASF (Germany), an epoxy-styrene-acrylic oligomer with a weight average molar mass, M_w, of 6800 g/mol and a T_g at 54°C, was supplied in the form of granules.

Methods

Processing of the PLA/MFC biocomposites

PLA/MFC biocomposites containing 1 (P1) and 5 (P5) wt% MFC were prepared using a two-step solvent-free process. A wet mixing procedure based on a papermaking process was first used to combine the PLA powder and the MFC. This was followed by a compression molding step and, in certain cases, an extrusion step.

In the wet mixing procedure, the required amount of the MFC slurry was added to 50 ml of distilled water and stirred for 30 s with a hand blender. The PPLA were then added to the MFC suspension and stirred for 1 min. Dewatering was performed by passing the mixture through a glass filter (ROBU Number 4) equipped with a porous membrane (Millipore type DVPP, 0.65 µm). The filtrate was dried under vacuum (200 mbar) at 35°C for 3 h and then at 65°C for 16 h, and placed in a circular mold with a diameter of 80 mm (the same diameter as the filter) and a thickness of 2 mm. Compression molding was carried out at 195°C for 10 min using a hydraulic press (Fontjine, Netherlands) to apply a 6 kN load to the preform during both heating and subsequent cooling. The resulting circular plaques weighed approximately 13 g. For comparison, neat PLA preforms (P0) were produced by the same process, i.e. mixing with water, dewatering, drying and compression molding.

Certain of the neat PLA (P0) and composite preforms (P1 and P5) were cut up, dried and extruded, in order to investigate the influence of the extrusion step on the MFC dispersion and the associated physical properties. Extrusion was carried out at 180°C (melt temperature, 175°C) using a microextruder equipped with twin conical co-rotating screws and a capacity of 5 cm³ (DSM Micro 5 compounder, Netherlands). The screw rotation speed was 125 r/min and the residence time was 180 s. The relatively low extrusion temperature was chosen to minimize degradation of the cellulose.^23,24 The extrudates containing 0, 1 and 5 wt% MFC will be referred to as EP0, EP1 and EP5, respectively.

Where required, chain extender was added to the 8052D GPLA at a concentration of 1.5 wt% and to P0, P1 and P5 during the extrusion step. Following the manufacturer’s recommendation, the residence time was set to 120 s, the extrusion temperature to 200°C (melt temperature, 195°C) and the screw speed to 125 r/min. GPLA, P0, P1 and P5 extruded with Joncryl will be referred to as JPLA, J0, J1 and J5, respectively. Table 1 summarizes all the materials used in this study, along with the corresponding processing methods.

Table 1.

List of the materials used in this study and the processing steps applied

Name	Materials				Wet mixing + compression moulding	Extrusion	Addition of 1.5 wt% Joncryl
	8052D granules	8052D particles	MFC wt%
	8052D granules	8052D particles	1	5
GPLA	X	-	-	-	-	-	-
PPLA	-	X	-	-	-	-	-
P0	-	X	-	-	X	-	-
P1	-	X	X	-	X	-	-
P5	-	X	-	X	X	-	-
EP0	-	X	-	-	X	X	-
EP1	-	X	X	-	X	X	-
EP5	-	X	-	X	X	X	-
JPLA	X	-	-	-	-	X	X
J0	-	X	-	-	X	X	X
J1	-	X	X	-	X	X	X
J5	-	X	-	X	X	X	X

MFC: microfibrillated cellulose; GPLA: PLA in the form of granules; PPLA: PPLA: PLA particles.

Processing of the PLA/MFC biofoams

ScCO₂ foaming made use of a high-pressure chamber integrating a control system allowing precise monitoring of the foaming parameters (SITEC Sieber Engineering AG, Switzerland). Specimens of the various materials (3 g) were placed in cylindrical open molds of 24 mm in diameter and inserted into the pressure vessel. The CO₂ pressure and temperature were then increased to the final saturation temperature, T_sat, and saturation pressure, p_sat, which were maintained for a certain time, t, according to previous studies on PLA foaming.^25,26 In the present case, p_sat was set to 200 bar for all the tests, and T_sat to 155°C, 165°C (t = 10 min) or 190°C (t = 20 min) for the chain-extended materials. Cell nucleation and growth (foam expansion) were initiated by the thermodynamic instability created by depressurization. Depressurization rates, dp/dt, of 4 bar/s and 14 bar/s were used here. Stabilization of the foam structures resulted from cooling induced by the pressure release and maintained by the cooling system of the chamber. Figure 1 summarizes the foaming parameters used for the different materials. A minimum of five foams were prepared for each set of conditions and for each material.

Viscosimetry

Viscosimetric measurements were performed using an Ubbelohde capillary viscosimeter (Lauda PVS 1/1 Dilution Viscometer UG 098, Germany) to determine the evolution of the molar mass of the neat PLA during the different processing steps. Specimens were prepared by dissolving 0.25 g of the PLA in 50 ml of chloroform. The intrinsic viscosity, [η], was measured at 25°C ± 0.01° and the viscosity average molecular weight calculated from the Mark-Houwink equation (1):

\bar{M_{v}} = \sqrt[α]{\frac{η / 100}{K}}

(1)

where the Mark-Houwink constants, K and α, are specific to a given system. Their values for poly(D,L-lactide) at 25°C in chloroform are K = 2.21 × 10⁻⁴ dl/g and α = 0.77.²⁷ A minimum of four specimens was analyzed for each material and each processing step.

Characterization

The dispersion of MFC in the preforms and in the extruded composites was determined by preparing 1 µm thick sections with an ultramicrotome (Reichert-Jung Ultracut-E) equipped with a diamond knife (Diatome) for optical microscopy (Olympus BH-2) in phase contrast mode.

The density ρ* of the foams was obtained by dividing their measured weight by their volume. Their compression behavior was investigated at room temperature using a Universal Testing Machine (UTS Test System, Germany) equipped with a 1000-N load cell. The measurements were made at a crosshead speed of 0.5 mm/min on cylindrical specimens with a height of 10 mm and a diameter of 20 mm machined from the foams with their axis parallel to that of the autoclave molds. The compression modulus $E_{L}^{*}$ was evaluated from the initial linear region of the stress–strain curves. Three to five specimens were tested for each material and each set of processing conditions.

Selected specimens were sectioned with a razor blade and carbon coated for observation by scanning electron microscope (SEM, Philips XLF-30, Netherlands) with an accelerating voltage of 3 keV and a working distance of 10 mm.

Results and discussion

MFC dispersion in the composite preforms

Figure 2(a) shows the morphology of P5. The MFC was localized in coarse bundles that formed a continuous network throughout the molding, with a cell diameter commensurate with the original PPLA particle diameters of 15 to 150 µm. The as-received MFC is known to consist of nanosized cellulose fibrils which show strong physical interactions mediated by hydrogen bonding.^28,29 During the wet mixing and drying steps, the MFC is forced to the interparticle boundaries and the resulting network stabilized by the interfibrillar interactions. The polymer clearly did not impregnate the MFC bundles during compression molding under the present conditions, and the network remained substantially intact. However, the more intense shear associated with the extrusion step was sufficient to break up the MFC network into fragments with lengths and thicknesses consistent with the original bundle size, as shown for EP5 in Figure 2(b). Transmission electron microscopy of ultrathin sections from composites produced by a similar processing route has indicated that the bundles visible in optical micrographs accounted for all the MFC present, i.e. that there was little re-dispersion of the MFC at the nanometric scale during extrusion.

Biocomposite foam morphologies

P1 and P5, compounded by wet mixing and compression molding, underwent far more limited and local expansion during foaming (Figure 3(c) and (d)) than the neat materials P0 and GPLA (Figure 3(a) and (b)). This was attributed primarily to the stability of the MFC network (cf. Figure 2(a)), which is expected to remain rigid at the foaming temperature and hence impede expansion of the polymer.²⁹ Reduced expansion of wood fiber polymer foams may also be linked to the high degree of cristallinity of cellulose,³⁰ so that a continuous MFC network may also hinder gas diffusion into the matrix.³¹^–³⁴ Moreover, investigations of wood fiber composites based on polyethylene (PE) and polyvinylchloride (PVC) suggest that the poor interface between fibers and matrix provides a channel through which dissolved gas can escape on pressure release, without expanding the matrix.^12,31,35,36 As shown in Figure 4, the foam expansion was five to six times greater in foams obtained from the composites after extrusion. The loss of continuity of the MFC network (cf. Figure 2(b)) is expected not only to reduce its overall mechanical stiffness but also its effectiveness as a diffusion barrier and disrupt easy gas diffusion paths along the PLA-MFC interfaces, consistent with any of the above explanations for the low expansion in the un-extruded composites.

Representative SEM micrographs from the foams are shown in Figure 5. The cellular structure was relatively homogeneous in both the neat PLA (GPLA and EP0) and the extruded composites (EP1 and EP5). Decreasing T_sat and dp/dt resulted in reduced cell diameters and a narrower cell diameter distribution for all the materials, so that the most homogeneous structures were obtained with T_sat = 155°C and dp/dt = 4 bar/s. Coalescence was more in evidence at high T_sat and high dp/dt, resulting in significantly increased cell diameters locally. The relatively low viscosity associated with these conditions was assumed to promote thinning of the cell walls and hence facilitate the gas diffusion. In the absence of rapid stabilization, which is controlled by the cooling rate during depressurization, coalescence and open cell structures are favored.^31,35,37

It is apparent from Figure 5 that coalescence became marked as the number of processing steps and the MFC content increased. Thus, the neat GPLA foams showed a more homogeneous structure than the EP0 foams, which in turn showed a more homogenous structure than the composites. A contributing factor may be decreases in viscosity owing to degradation of the PLA, which would also have a significant impact on the final foam properties. This was investigated by using intrinsic viscosimetry to estimate the viscosity average molecular weight, $\bar{M_{v}}$ , for the GPLA, PPLA, P0 and EP0, and hence assess its role in the influence of the different processing steps on the evolution of the melt viscosity and the mechanical properties.¹⁴^–^16,38 The results are shown in Figure 6. The as-received GPLA had the highest molecular weight, $\bar{M_{v}}$ = 92,500, and processing to form the powder (PPLA) resulted in only a slight decrease ( $\bar{M_{v}}$ = 89,400). On the other hand, $\bar{M_{v}}$ for the wet mixed neat PLA preforms decreased to 82,800, and there was a further decrease to 74,000 after the extrusion step (EP0), i.e. a total decrease of about 20% with respect to that of GPLA. This could be attributed to the successive heating cycles above the melting temperature coupled with the presence of water during the mixing process. Moreover, degradation is expected to be even more pronounced in the presence of the highly hydrophilic MFC, although this could not be tested directly by viscosimetry.

Addition of a chain extender provides a means to increase the molar mass of PLA and hence its viscosity, melt strength and foamability. As shown in Figure 7, this led to much finer, more homogeneous structures in all the materials in spite of the higher T_sat used in this case, and the additional processing step, which would be expected to contribute to degradation of the neat PLA. Moreover, the cell size was significantly reduced in the composite foams. As discussed previously for the un-modified foams, this may be due not only to the influence of the MFC on viscosity and gas diffusion but also enhanced cell nucleation in the presence of the MFC.^39,40 The use of the chain extender therefore allows one to counteract the effects of degradation resulting from the multiple processing steps necessary to produce composite foams.

Foam mechanical properties

The compression moduli $E_{L}^{*}$ and the densities ρ* of the extruded and foamed materials without the chain extender are given in Figure 8 for the different processing conditions. The influence of molar mass degradation on the foam properties is clear from comparison of the data points for GPLA and EP0 with similar foam densities (cf. the black arrow in Figure 8 for T_sat = 155°C and dp/dt = 4 bar/s). It follows that the influence of MFC on the compression modulus could only be assessed directly from comparison of EP0, EP1 and EP5, i.e. for the same processing history. For these materials, the compression modulus of the foams increased from around 20 MPa to 50 MPa as the MFC content was increased from 0 to 5 wt%, regardless of T_sat and dp/dt. However, the foam densities also increased from around 0.15 to 0.35 g/cm³ in the presence of the MFC, as indicated by the grey arrow in Figure 8 for T_sat = 155°C and dp/dt = 4 bar/s. It should be emphasized that this density increase could not be explained directly by the presence of the MFC, because the density of cellulose is only slightly higher than that of the PLA matrix (1.5 g/cm³ for the cellulose fibrils as opposed to 1.25 g/cm³ for the PLA).⁴¹ It was rather a consequence of factors referred to previously, such as the influence of the MFC on viscosity, gas diffusion and cell nucleation, and hence on the final foam structure. Regardless of their origin, the observed density increases in the composite foams were assumed to account substantially for the increases in compression modulus, based on the model of Gibson and Ashby.⁴² This was also observed in polylactide/hydroxyapatite nanocomposite foams processed with supercritical CO₂ in a batch process. The role of any mechanical reinforcement by the MFC and the extent to which it is offset by increased PLA degradation in the presence of the MFC was less clear, given the experimental scatter in the data. This will be addressed in more detail in future work.

While, as stated previously, neither T_sat nor dp/dt appeared to have a significant effect on the density and compression modulus of the foams globally. However, the excessive coalescence observed for the highest T_sat and dp/dt, particularly for EP5, was reflected by both low densities and somewhat lower moduli than foams with comparable densities produced under other conditions. This was imputed to the correspondingly inhomogenous foam structures obtained in this case (cf. Figure 5).

The foam densities were found to be some 60 to 75% lower for the materials incorporating the chain extender, reflecting the improvements in foaming characteristics referred to in the context of Figure 7. Thus, densities of 0.037, 0.045, 0.069 and 0.13 g/cm³ were measured for JPLA, J0, J1 and J5 and the corresponding compression moduli were 2.7, 5.2, 5.1 and 27.3 MPa, respectively. While these values for the compression moduli were very much lower than for the denser foams obtained without the chain extender, they remained consistent with the overall dependence of modulus on density suggested by Figure 8.

Conclusions

PLA was successfully compounded with MFC using a wet compounding process. In the compression molded composite preforms produced by this method, the MFC formed a continuous network at the scale of the original PLA powder, stabilized by hydrogen bonding. During physical foaming, this network was found to impede significantly expansion of the polymer. However, the MFC network was weakened during subsequent extrusion, and this additional processing step was found to be crucial for the successful foaming of the composite preforms. The influence of the foaming parameters on the morphology and properties of the composite foams was also investigated. A relatively low saturation temperature, T_sat, of 155°C and depressurization rate, dp/dt of 4 bar/s gave the most homogenous cellular structures. For fixed T_sat and dp/dt, a less homogenous structure containing coalesced cells was observed for PLA/MFC than for unmodified PLA. The presence of the hydrophilic MFC is argued to accelerate hygrothermal degradation of the PLA during processing, resulting in a lower viscosity and thus poorer foamability. The compression modulus of the resulting foams was found to be sensitive to the foam density in all the materials, but also to degradation of the PLA as the number of processing steps increased. The use of a chain extender was shown to be effective in offsetting the effect of molecular degradation, allowing foams with significantly lower densities to be produced. The next stage of this work will focus on reducing PLA degradation and improving the MFC dispersion in order to optimize the performance of the final foams.

Footnotes

Funding

This study was partially funded by the SustainComp project (grant no. 214660) in the Seventh Research Framework Programme (FP7) of the European Union (EU).

Acknowledgments

The authors would like to thank Mikael Ankerfors and Dr. Fredrik Berthold at Innventia AB, Sweden, for providing the neat MFC as well as technical advice. Dr. David Eglin and Markus Glarner from AO Foundation in Davos are warmly acknowledged for intrinsic viscosity measurements.

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