Processing of Sustainable Cellular Biocomposites

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.

Processing of fibre and porosity gradients in cellular thermoplastic composites

Manuel Bühler

The aim of this study has been to develop processing techniques for novel bioresorbable thermoplastic cellular composites, reinforced with continuous inorganic fibres, for use as biodegradable scaffolds for bone tissue engineering. As the implanted composite scaffold starts to degrade, it should be progressively replaced by natural bone tissue, which should consequently support increasing proportions of the applied mechanical loads. In order to simulate natural high performance porous structures such as bone or light wood, composites with simultaneous gradients in fibre content and porosity were studied. In order to carry out investigations over wide ranges of fibre content and porosity and to ensure biocompatibility and bioresorbability, it was decided to focus on foamable poly(L-lactic acid) (PLA) and continuous glass fibres. As well as improved mechanical reinforcement due to the high aspect ratio, the use of continuous glass fibres offers the possibility of achieving higher loadings than with short fibres or particles, increasing the potential range of mechanical properties. For processing the composites, a fibre winding technique developed in house was used to prepare unidirectional preforms with gradient structures tailored at the fibre bundle level. PLA fibre bundles, containing 36 monofilaments were prepared on a spinning line, and mingled with glass fibre bundles comprising 250 or 1000 monofilaments. During the winding process, the PLA fibres were intimately mixed with the reinforcing fibres, to ensure a good impregnation during subsequent processing steps. Moreover it was possible to maintain the relative positions of the fibres during pre-consolidation of graded composites. The resulting preforms had a porosity of 25% and could be further consolidated to decrease the porosity, or foamed in an autoclave to obtain cellular composites (foams) with porosities of up to 90%. It was observed that, independently of the applied foaming conditions, the presence of continuous fibres reduced the porosity by an amount equivalent to twice the fibre volume fraction. This processing method provided composites with a simultaneous gradient in fibre content and porosity. In order to predict the Young's modulus and the collapse strength of the cellular composites a model was proposed, which combines the buckling of composite columns within a porous structure with the property predictions for neat polymer foams. In neat foams with a porosity of 60%, the Young's modulus was calculated to be 0.25 GPa. By adding a fibre volume fraction of 14% the modulus reached 1.2 GPa. The respective collapse strengths were 5 MPa and 16 MPa. Experimental results were consistent with the predictions and validated the model. The model determines the fibre content required for composites with a given porosity and stiffness. Gradient cellular composites were obtained with a range of porosity between 50% to 90% and a range of stiffness between 100 and 1500 MPa. Furthermore the processing parameters can be adjusted to maintain a desired porosity when fibres are added. Thus, composites that fulfill the mechanical requirements for bone scaffolds can be achieved in a rational manner. The long term behaviour of the foams was tested under physiological conditions. Viscoelasticity tests provided evidence for the important role of the unidirectionally oriented fibres. Whereas the cells in neat foams collapsed after at most two days when loaded at 0.69 MPa, the fibre reinforced foams resisted longer and their creep strain was well described by a power law. The fatigue properties were compared to reported values of natural trabecular bone. None of the neat foams with a porosity of 74% resisted to the first load cycle of 7.3 MPa. With a fibre volume fraction of 11% and a comparable porosity of 72%, the composite scaffolds resisted for at least 11'000 loading cycles. To compare, trabecular human bone failed after 1909 cycles, when loaded with 8.5 MPa. It was concluded that the reinforced composites have the potential to replace natural bone. Preliminary in vitro results with human foetal osteoblasts and in vivo tests on femurs of rats were promising because the cells proliferated well on the composite substrates. Further investigations for applications in bone tissue engineering can be planned. Furthermore, the studied process and the obtained results are envisaged for other material systems to prepare cellular composites for novel lightweight applications in transportation and building industry.

EPFL2008

Cellular thermoplastic composites with microstructural gradients of fibres and porosity

Pierre-Etienne Bourban, Manuel Bühler, Jan-Anders Månson

New thermoplastic composites with their composition tailored at the yarn level were proposed and successfully prepared. A specific processing method was developed for composites exhibiting different fibre and porosity concentration gradients. Mechanical properties of such composites were determined by three-point bending tests. In order to carry out investigations on wide ranges of fibre and porosity fractions, a material system based on poly(lactic acid) (PLA), and continuous glass fibres was processed into a composite foam structure with up to 85% porosity and a modulus reaching 1.5 GPa. Micrographs confirmed that the fibre placement could also be controlled at a microscale as well as at the usual millimetre range. Furthermore it was shown that porosity distribution is indeed influenced by the presence of continuous reinforcing fibres. Gradient cellular composites are envisaged for bone replacement and for lightweight sandwich structures.

2008

Hybrid Polymer Foams for Osteochondral Tissue Engineering

Matthieu Cuénoud

Articular cartilage is a soft tissue that covers the ends of articulating bones, and is essential for maintaining joint motion, i.e. translation and rotation between the bones. Articular cartilage nevertheless has a limited capacity for self-repair, and cartilage lesions following traumatic injury may consequently initiate an irreversible degenerative process. This ultimately leads to progressive loss of joint function, pain and medical costs. Many different clinical treatments have been investigated, but these have not so far led to satisfactory long-term solutions. Tissue engineering research has therefore focused on the preparation of synthetic bioresorbable scaffolds that provide a template for cells and guide the process of tissue regeneration. In the present study, it is proposed to develop an osteochondral scaffold using novel solvent-free processing methods based on supercritical carbon dioxide (CO2) foaming. The approach is first to tailor the mechanical properties of medical grade polylactides (PLA), using poly(ethylene glycol) (PEG) as a plasticizer, to produce scaffolds with reduced elastic moduli intended for cartilage repair. These polymer scaffolds are then combined with an existing biocomposite scaffold (PLLA/bioceramic), developed for bone repair, to give a bilayered scaffold suitable for osteochondral repair. The investigation thus covers: (I) the preparation of plasticized polylactide and the dependence of its properties on composition; (II) the design of processing windows for the production of porous plasticized polylactide scaffolds by supercritical CO2 foaming; (III) the preparation of hybrid structures with gradients in composition and properties, and tailored interfaces; (IV)the properties and in vitro response of sterilized scaffolds. Plasticization of semicrystalline poly-L-lactide (PLLA) with PEG reduces Tg to below 37 °C, depending on the composition, implying the amorphous regions to be in the rubbery state at body temperature. While immersion in an aqueous medium tends to promote phase separation in blends containing low molar mass PEG, relatively good stability is observed with PEG with a molar mass of 35,000 g/mol. It has thus been possible to prepare interconnected, open-pore foams with porosities of more than 75 %, cell diameters in the range 200 to 700 pm and compression moduli of a few MPa. It was also observed that addition of PEG resulted in foams with increased pore diameters and lower foam densities than the neat polylactide, but also increased pore coalescence at high saturation pressures, effects that were accounted for by the large decrease in melt viscosity on addition of PEG. Furthermore, the compression moduli of the foams were shown to be strongly reduced on addition of PEG, which was assumed to be a direct consequence of the plasticization of the polylactide by PEG and the accompanying decrease in glass transition temperature, implying the amorphous content to be in the rubbery state at in vivo temperature, as required. Thus, foams with moduli ranging from 0.3 to 15 MPa could be prepared from the different PLA/PEG blends and copolymers studied. Bilayered scaffolds were produced by assembling the plasticized foams with a biocomposite foam using an adapted fusion bonding method. This allowed control of interfacial properties such as the bonding strength, and the interface density and permeability. These hybrid scaffolds exhibited well-defined layers with suitable properties for the repair of bone and cartilage, respectively. Finally, X-ray and y irradiation was shown to strongly degrade the polymer matrix. However, it was found that treatment with ethylene oxide gas could be used to sterilize the scaffolds without significantly modifying their properties, or compromising their biocompatibility, as borne out by in vitro tests. The approach proposed in this work was hence concluded to be highly promising for cartilage repair, and the resulting hybrid osteochondral scaffolds are ready for the next stage of the developmental work, which will consist of extended in vivo studies.