Static and dynamic mechanical properties of nanosilica and multiwalled carbon nanotube reinforced acrylonitrile butadiene styrene composites: theoretical mechanism of nanofiller reinforcement

Experimental and theoretical approaches were employed to analyze the static and dynamic mechanical properties of acrylonitrile butadiene styrene (ABS) nanocomposites reinforced with multiwalled carbon nanotube (MWCNT) and nanosilica. Enhanced stiffness and tensile strength were observed for composites with 5% (wt) CNT and 3% (wt) nanosilica. Micromechanical models were highly promising to predict the experimental Young's modulus by 2% (wt) of nanofiller content, beyond which it deviated due to the nanofiller population and the reduction in interparticle distance. At the optimum content of 5% (wt) MWCNTs in the ABS matrix, enhanced storage and loss moduli and lowered damping peak were observed, which was attributed to the immobilization of the segmental polymer chains. From the DMA parameters, the dynamics of the polymer chain were investigated and the fraction of constrained regions and entanglement density were quantified. These findings demonstrated that composite with higher constrained regions and entanglements exhibited superior mechanical performance. Amongst all composites, 5% (wt) MWCNT-reinforced ABS composite showed an increment in reinforcing efficiency, entanglement density, and constrained regions by 195.6%, 115% and 55.5%, respectively, with regard to 3% (wt) nanosilica-reinforced ABS composite. The effectiveness of dispersion and interfacial adhesion of CNTs with ABS was improved by carboxyl treatment and functionalization with ABS-g-MaH compatibilizer with regard to silane-treated nanosilica/ABS composites. Combined analysis of microstructure, tensile properties and dynamic mechanical parameters such as entanglement density, effectiveness of filler, constrained volume of polymer chains and adhesion factor demonstrated the effectiveness of high aspect ratio carboxyl-treated MWCNTs as a better reinforcing agent in comparison with nanosilica in ABS matrix.

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

Ultra-light photovoltaic composite sandwich structures

Julien Rion

The ultra-light photovoltaic sandwich structure is a new multifunctional structure concept enabling weight and thus energy to be saved in high-tech solutions such as solar cars, solar planes or satellites. The novelty of this approach is to use solar cells as a load carrying element in the structure. The aim of this work was to investigate the failure mechanisms of such ultra-light sandwich structure and their correlation with microstructure, processing pressure, and strength in order to obtain optimal design and processing. To this end, composite sandwich structures were extensively studied with weights in the range of 650 – 850 g/m2, and comprising one 140 µm thick skin made of 0/90° carbon fiber-reinforced plastic (CFRP), one skin made of 130 µm thick mono-crystalline silicon solar cells, and a 29 kg/m3 honeycomb core. As a first step, core-to-skin bonding in a symmetric (CFRP / core / CFRP) sandwich, for which a design criterion was lacking, was especially studied. An adhesive deposition technique was developed enabling the adhesive weight used for core-to-skin bonding to be tailored. Based on adhesive contact angles, the formation of the adhesive fillets between honeycomb cell walls and skin was modeled. Core / skin debonding energy was measured and compared to core tearing energy measured with a new video-based method, and the failure mechanisms during skin peeling were investigated. It was thus ascertained that, to provide the highest debonding energy-to-weight ratio, the optimal adhesive weight was 35-40 g/m2. Furthermore, in contrast with classic sandwich structures with thicker skins, it was observed that the bending strength of the ultra-light sandwich panels increased with adhesive weight. This was due to the formation of adhesive fillets, which significantly increased the bending stiffness of the thin CFRP skin, and thus increased the compressive load causing local instability of the skin. Models taking into account the increased skin stiffness showed that the best adhesive quantity required to increase the strength-to-weight ratio was ∼40 g/m2. In a second step, the influence of processing pressure on the morphology and strength of symmetric (CFRP / core / CFRP) ultralight sandwich structures was investigated by using one-shot vacuum bag processing. This showed that higher processing pressures caused the formation of larger adhesive fillets and an increased waviness of the CFRP skin on vacuum bag side. These two effects had conflicting impacts on the strength of the structure. Waviness of the skin favored local instabilities, whereas adhesive menisci stabilized the skin. Modeling of the local instability of the skin by taking into account the waviness of the skin and the size of the menisci as a function of processing pressure enabled an optimal processing pressure of 0.7 bar to be identified, giving the highest strength-to-weight ratio. The third step of the study was devoted to the mechanical analysis of the mono-crystalline silicon solar cells. The brittle behavior of the cells was confirmed, and the Weibull failure probability curve was established with the mean tensile strength at 221 MPa. It was demonstrated by experimental testing and finite element modeling (FEM) that the low strength of the cells compared to the intrinsic strength of silicon (∼6.9 GPa) was not due to surface texturation of the cells used for increased efficiency, but to more severe surface or edge defects. FEM also showed that no significant reinforcing effect of the cells could be obtained with polymer encapsulation. In addition, thermo-mechanical stresses due to a mismatch of the coefficient of thermal expansion (CTE) between the Si cells and the polymer encapsulation were found to be negligible. In order to protect the cells against the environment, encapsulation of the cells was successfully carried out, using highly transparent fluoropolymer films treated with SiO2 plasma sputtering for improved adhesion, together with silicone adhesive. Finally, the integration of solar cells as a photovoltaic skin of an ultra-light sandwich structure was achieved using thin stress transfer ribbons to ensure load transfer between adjacent cells. It was observed that the cells were not damaged by sandwich panel processing, even in curved panels, thus showing that the processing windows of the different constituents were compatible. The asymmetric (Si / core / CFRP) photovoltaic sandwich structure with a weight equal to 800 g/m2 and a specific power density equal to ∼250 W/kg (i.e. 20 times more than standard commercial photovoltaic panels) demonstrated an equilibrated mechanical behavior, i.e. the CFRP skin, reinforcing ribbons, and solar cells had similar failure loads.

EPFL2008

Self-Healing Fiber-Reinforced Composites with Tailored Supramolecular Matrices

Federica Sordo

Fiber Reinforced Composites (FRCs), due to their intrinsic heterogeneity, are generally affected by complex multi-scale failure mechanisms leading to the formation of matrix cracks that are difficult to detect and repair. FRCs with the ability to heal and recover at least part of their initial properties after damage could present a solution to extend their lifetime and reliability. A possible approach is to introduce external healing agents in the matrix of the composite, through capsules or micro-channels, but this is often difficult to achieve in a high volume fraction reinforcement composite. An alternative approach is to rely on intrinsic self-healing polymers as matrices, which alleviates the need for additional phases beyond matrix and reinforcement. The present work investigated this second approach, by exploring for the first time the feasibility of processing glass fabric reinforced composites based on supramolecular hybrid network self-healing matrices and analysing their resulting properties and transfer of the self-healing ability to the composite. Hybrid networks based on epoxy chemistry, combining covalent cross-links and cooperative reversible hydrogen bonds, were selected as benchmark materials. These systems were charac- terized, in particular in terms of processing parameters and self-healing ability, and showed to be a-priori compatible with FRCs manufacturing techniques and demonstrated higher healing efficiency by increasing the H-bonds content in the polymer network, from none to 50%. On the other hand, networks characterized by large number of physical cross-links exhibited limited solvent and creep resistance, hindering their potential application. These issues were addressed by modifying the average functionality of the monomers through the combination of bifunctional and tetrafunctional epoxy resins in the compositions while keeping the use of a catalyst, leading to better controlled networks, both in terms of kinetics and final structure, and to propose a range of materials with several levels of compromise between self-healing efficiency, mechanical strength, creep resistance, and damping properties. In particular, 50% to 100% self-healing recovery of tensile properties were observed after one day, depending on the tetraepoxide content. In parallel, since the Tg of benchmark supramolecular hybrid net- works was in the range from 11-16 âŠC, an aliphatic diepoxide was introduced in the synthetic procedure to propose materials with a Tg down to -10 âŠC, to reach applications closer to those of elastomers. In view of FRCs development, the ability of partially supramolecular self-healing polymers to bond to inorganic reinforcements and to heal interfacial failure was then investigated: two dedicated butt joint-like and tack-like test methods were developed. Adhesion strength of the polymers cured on glass substrates decreased with increasing H-bonds content in the polymer networks (from 0 to 50 %) as the mechanical strength of the polymer also decreased, while the failure mechanism shifted from adhesive to cohesive due to the possibility to form hydrogen bonds with the glass substrates. Interface restoration through healing of the polymer matrices however increased from none to a strength recovery up to 70 % after 1 h healing time for the 50% H-bond polymer...

EPFL2016