Recycled Glass Fiber Composites from Wind Turbine Waste for 3D Printing Feedstock: Effects of Fiber Content and Interface on Mechanical Performance

This research validates the viability of a recycling and reusing process for end-of-life glass fiber reinforced wind turbine blades. Short glass fibers from scrap turbine blades are reclaimed and mixed with polylactic acid (PLA) through a double extrusion process to produce composite feedstock with recycled glass fibers for fused filament fabrication (FFF) 3D printing. Reinforced filaments with different fiber contents, as high as 25% by weight, are extruded and used to 3D print tensile specimens per ASTM D638-14. For 25 wt% reinforcement, the samples showed up to 74% increase in specific stiffness compared to pure PLA samples, while there was a reduction of 42% and 65% in specific tensile strength and failure strain, respectively. To capture the level of impregnation of the non-pyrolyzed recycled fibers and PLA, samples made from reinforced filaments with virgin and recycled fibers are fabricated and assessed in terms of mechanical properties and interface. For the composite specimens out of reinforced PLA with recycled glass fibers, it was found that the specific modulus and tensile strength are respectively 18% and 19% higher than those of samples reinforced with virgin glass fibers. The cause for this observation is mainly attributed to the fact that the surface of recycled fibers is partially covered with epoxy particles, a phenomenon that allows for favorable interactions between the molecules of PLA and epoxy, thus improving the interface bonding between the fibers and PLA.

Fiber reinforced polypropylene nanocomposites

Chrystèle Houphouet-Boigny Favre

The aim of this thesis is to assess the feasibility of integrating nanoparticles into glass fiber (GF) reinforced isotactic polypropylene (iPP) composites via existing thermoplastic processing routes, and to investigate whether this results in significant improvements in the mechanical properties of the final composites. A longer term aim will be to extend the approach to the preparation of hybrid composites with added non-structural functionality. However, the nanoparticles that have provided the focus for the present project, montmorillonite layered silicates (MMT) and nanocarbons, were chosen for their potential as structural reinforcing elements. A melt-spinning grade and a film grade of iPP were used to prepare iPP-based nanocomposite precursors in the form of melt-spun fibers and extrusion-calendered films respectively. Long glass fiber (LGF) and glass mat thermoplastic (GMT) composites were then compression-molded from co-woven, co-wound and intercalated semi-finished products. The processing behavior and structural performance of the resulting composites are discussed in terms of the matrix morphology and its influence on the matrix rheological and mechanical properties, and interactions between the matrix and the reinforcing fibers. The nanocomposites were prepared by either (i) combined solvent and melt-mixing or (ii) direct melt-mixing. Combined solvent and melt-mixing was more suitable for dispersing carbon nanofibers (CNF), which tended to agglomerate. With iPP/MMT, both routes gave a mixed intercalated-exfoliated morphology with an MMT interlayer spacing up to 57 % greater than in the as-received MMT. However, direct melt-mixing was considered to be better suited to industrial requirements, and also more convenient for laboratory scale preparation. Melt-compounded iPP/MMT injection moldings showed a monotonic increase in stiffness with increasing MMT content, a 40 % increase in tensile modulus being measured at 13.5 wt% MMT, for example. The tensile strength, on the other hand, reached a maximum 10 % increase over that of the pure iPP at about 3 wt% MMT, but fell off at higher MMT contents. iPP/MMT and iPP/CNF fibers were melt-spun using a laboratory-scale industrial spinning line. Processability was consistent with the melt rheology, the maximum MMT content for which fiber spinning was possible being about 5 wt%. The MMT platelets were aligned with the fiber axis over the whole range of MMT loadings and fiber draw ratios. MMT particle aspect ratios of about 150 were observed by TEM in this case, i.e. greater than in the as-compounded iPP/MMT, for which the particle aspect ratios were about 50. An aspect ratio of 150 was found to be consistent with micromechanical modeling of the observed increases in fiber stiffness with MMT content, which reached 170 % for iPP/2 wt% MMT fibers melt-spun with a draw ratio of 1 and a drive-roll velocity of 360 m/min. The tensile strength again reached a maximum at about 3 wt% MMT. The thermal stability of the fibers, determined by thermal mechanical analysis, also increased on MMT addition, the onset of extensive fiber creep shifting from 90 °C for pure iPP fibers to about 110 °C for iPP/1.1 wt% MMT fibers. Moreover, significantly reduced shrinkage was observed in the presence of MMT, which is a potential advantage for textile based composite processing. In the case of iPP/CNF fibers, limited particle orientation and the presence of aggregates in all the formulations led to relatively poor tensile properties, about 50 % lower than those obtained with iPP/MMT fibers melt-spun with the same filler content (4 wt%) and processing conditions. iPP/MMT therefore provided the main focus for subsequent work. iPP/MMT films were produced by extrusion-calendering. Partial orientation of MMT platelets in the melt flow direction resulted in anisotropic stiffness and strength. However, both the tranverse and axial stiffnesses increased with MMT content, with improvements of up to 75 % at 5.9 wt% MMT with respect to those of the pure iPP films, consistent with Halpin-Tsai predictions for composites containing oriented platelets with the observed aspect ratio of 50. The fracture resistance of the films, determined using modified essential work of fracture (EWF) tests, was likewise strongly dependent on the testing direction. For axial crack propagation, the EWF decreased monotonically with MMT content, but for transverse crack propagation, it reached a maximum at about 3 wt% MMT. This was attributed to orientation dependent cavitation and crack deviation in the presence of the MMT particles. Rheological measurements indicated increases in the low shear rate melt viscosity by up to two orders of magnitude on MMT addition to the iPP, with a potentially significant influence on the impregnation behavior of composite preforms. For co-woven and co-wound LGF-matrix fiber preforms, impregnation was highly dependent on the fiber bed geometry. For 40 vol% glass fiber co-wound composites, the porosity increased from about 7 vol% for a pure iPP matrix compression molded at 0.6 MPa, to 14 vol% for a iPP/3.4 wt% MMT matrix compression molded at 1.8 MPa. However, the more intimate mixing between the fibers obtained in co-woven preforms led to more consolidated composites in each case (below 2 vol% porosity) with no filtering of the MMT particles. In modeling the impregnation kinetics of glass mat thermoplastic composites (GMT) based on iPP/MMT, the matrix was therefore considered to behave as a continuum and, for simplicity, to show Newtonian behavior. Consistent results were obtained, but in the presence of MMT, higher impregnation times were predicted than observed experimentally in model GMT preforms, owing to the nonlinear response of the nanocomposites. Moreover, under experimental conditions corresponding to the industrial process (0.2 MPa at 200 °C), iPP/5.9 wt% MMT-based hybrid composites were fully impregnated (porosity between 11 and 17 vol%) and the glass mat completely relaxed after about 30 s of compression molding, which is consistent with typical GMT industrial process cycle times (20 - 60 s). Fully consolidated 30 wt% GF - hybrid GMT specimens were prepared for mechanical testing by compression molding at 2 MPa and 200 °C for 10 min, resulting in a porosity of about 2 vol%. At room temperature, the flexural modulus and strength of iPP/MMT-based GMTs increased monotonically with MMT addition, the increases reaching about 45 % and 33 % respectively at 5.9 wt% MMT. The increase in the flexural modulus on MMT addition was greater than predicted on the basis of a conventional rule of mixtures taking into account glass fiber orientation and aspect ratio, and the measured Young's moduli of the matrix. This was tentatively attributed to improvements in the flexural properties of the matrix in the presence of the MMT higher than those observed in tension. Increases in the flexural modulus and strength were also observed at 50 °C and 90 °C, but were less marked than at room temperature. The impact strength of the hybrid composites decreased with increasing MMT content at room temperature owing both to the decrease in matrix fracture resistance inferred from the data for the precursor films, and to an improved fiber-matrix interface as reflected by SEM observations, and argued to be due to the presence of coupling agents. Given that the presence of MMT was shown not to have serious consequences for impregnation, taken as a whole these results are considered highly promising for the implementation of nanocomposite matrices in GMT, and to establish the general feasibility of producing hybrid fiber reinforced thermoplastic nanocomposites by conventional processing routes.

EPFL2007

Structural behavior and durability of multifunctional fiber-reinforced polymer components

Florian Riebel

In the context of sustainable development, building insulation represents a major concern as a means of increasing energy efficiency. In order to comply with new norms such as the Passive House standard, locations where load-bearing components must penetrate the building's insulating envelope, such as when cantilevered slabs (e.g. balconies) are anchored to building walls, constitute a particular challenge. With conventional steel reinforcement, thermal bridges are inevitable and insulation is weakened. To overcome this deficiency, a multifunctional joint has been developed in which GFRP (glass-fiber reinforced polymer) elements performthe necessary structural functions without compromising the continuity of the building's insulating envelope. This is due to GFRP materials' low thermal conductivity, which is magnitudes smaller than that of concrete and steel, making them particularly suitable. The anchorage of balconies requires linkages that can resist shear as well as tensile and compression forces in the upper and lower parts of the slab. In a first step, a hybrid joint was created in which the lower compression steel reinforcement was replaced by a compression-shear (CS-)element made of GFRP. This consisted of a short pultruded profile with cap plates bonded to its cut ends. In addition to compression forces, the element was intended to bear parts of the shear load. The joint was investigated in various full-scale beam specimens each representing a section of the slab. Based on the results, the joint's structural behavior was modeled analytically and structural requirements for the CS-element were determined. It was seen that shear transfer through the element increased with increasing shear-to-moment ratios. In a second step, an all-GFRP joint was created in which, in addition to the element in the compression zone, a tension-shear (TS-)element replaced the remaining steel reinforcement. This element consisted of the same pultruded profile cut to longer sections and penetrating the concrete. To anchor the tensile forces in the concrete, ribs were bonded to its surface. This joint type was also investigated through full-scale beam experiments similar to the hybrid-joint beams. The load transfer through the joint and into the concrete was studied and modeled analytically. The behavior of the all-GFRP joint was as ductile as that of the hybrid joints. The TS-element bore the main portion of the shear load independently of the shear-to-moment ratio however. The CS-element used in both joint types was of special interest with regard to remaining strength and stiffness after long-term service life. Exposure to alkaline concrete-pore solution represents a particular threat to the polyester matrix and glass fibers inside the GFRP material. Therefore, CS-elements were immersed in alkaline liquids at different temperatures and their compression strength and stiffness were studied during a period of eighteen months. Material degradation was investigated by SEM-microscopic images and EDX-analysis. The observed loss of compressive strength was ascribed to moisture diffusion and chemical degradation of the fibers, matrix, and fiber-matrix interface. It was shown that the strength degradation rate at different temperatures followed the Arrhenius rate law and that remaining strength after long time spans could therefore be projected by extrapolating the measurements. Remaining strength after 70 years of service life was found to be sufficient for the element never to become the critical failure location and classic concrete theory can be applied to verify the joint. Since stiffness at high temperatures was observed to already remain constant after short exposure times, this value could directly be extrapolated to apply to a 70-year service lifespan.

EPFL2006

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