Fiber reinforced polypropylene nanocomposites

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.

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

Highly filled polymer nanocomposite films derived from novel nanostructured latexes

Riccardo Ruggerone

The overall aim of this thesis has been to assess the potential of latex-based technologies for the preparation of polymer/clay nanocomposites. The key feature of latex-based technologies is that they offer the possibility of improved control of the final nanocomposite morphology at significantly higher clay loadings than can be obtained with more conventional processing techniques, such as melt blending or in situ polymerization. The idea is to exploit swelling of the clay in either the aqueous or the monomer phase of a water-based latex, depending on the clay surface functionalization, to produce hybrid polymer/clay latex particles with controlled diameters of the order of 100 nm, which may then be consolidated to produce solid nanocomposite films. The materials considered in this work were based on styrenic matrices, considered to be a model system, and acrylics, which are of more interest for commercial coating applications. Two different polymerization techniques were investigated, namely conventional emulsion polymerization and miniemulsion polymerization. The thermal and mechanical properties of films produced from the resulting latexes were then studied in detail. Conventional emulsion polymerization was found to be particularly suitable for the preparation of particles with a well-defined "armoured" morphology, in which the clay formed a more or less complete shell around a matrix core, providing the focus for the remainder of the project. Clay contents of up to about 50 wt % were obtained for both the styrenic and the acrylic latexes using this approach, with excellent degrees of dispersion, the average clay aggregate thickness not exceeding 10 nm. The armoured morphology of the latex particles resulted in a cellular arrangement of the clay in the consolidated films, which became better defined as the clay content increased. The reinforcing effect of the clay on mechanical properties varied according to the physical state of the matrix. Increases in Young's modulus by a factor of 3 to 4 were observed in styrenic films with the cellular morphology in the glassy state, and the degree of exfoliation of the clay was found to be a critical parameter under these conditions, samples containing 5 to 7 wt % of clay showing increased moduli with respect to those obtained at somewhat higher clay contents, for which aggregation was more apparent. In the rubbery state, on the other hand, the Young's modulus increased by more than 2 orders of magnitude for clay contents above 20 wt % and was strongly correlated with the overall filler content. Thermal analysis showed that a significant proportion of the matrix remained immobilized in the rubbery state, i.e. did not contribute to the glass transition. This was argued to be due to strong physical confinement of regions of the matrix intercalated in the clay aggregates. While the increases in Young's modulus in the glassy state could be accounted for in terms of classical micromechanical models, such as those of Halpin-Tsai and Mori-Tanaka, the same models failed to predict the behaviour in the rubbery state. Models based on foam mechanics were therefore developed incorporating an immobilized matrix fraction in the cell walls, whose elastic properties were treated as fitting parameters. Although somewhat different values of the Young's modulus for this immobilized matrix fraction were required to fit the data, depending on the details of the model, they were consistently found to be two to three orders of magnitude greater than that of the neat matrix in the rubbery state (but not to exceed the Young's modulus of the matrix in the glassy state), providing direct evidence for the importance of this interphase for the overall nanocomposite properties. The importance of the cellular arrangement of the clay was also confirmed through comparison with nanocomposites containing non-cellular morphologies resulting from other preparation techniques or the use of mechanical deformation to break-up the initial cellular structure. Finally, it was demonstrated that the results obtained for the styrenic systems could be extended to acrylic-based nanocomposites with comparable morphologies, underlining their broader significance for formulations of potential commercial interest. A further goal of this thesis was to study the effect of the clay on the microdeformation and fracture mechanisms of the nanocomposites, and it was also of interest to compare these results with those obtained for conventional isotactic polypropylene (PP)/clay nanocomposites prepared by melt blending, whose macroscopic properties have been studied previously in our institute. In situ TEM investigation of deformation in glassy styrenic nanocomposite films of about 200 nm in thickness containing the cellular structure revealed a decrease in local matrix drawability at clay contents above 10 wt %, and extensive coarse cavitation, thought to be associated with the particle cores, which replaced crazing as the dominant deformation mechanism, accounting for the observed decrease in macroscopic tensile strength at intermediate clay contents. Moreover, at the highest clay contents, these mechanisms were replaced by failure of the particle-particle interfaces, leading to an extremely brittle macroscopic response. Above Tg, localized deformation zones were also observed, indicating the network formed by the clay and the immobilized regions of matrix to show yielding behaviour, again consistent with the macroscopic response. The conventional PP/clay nanocomposites also showed a decrease in matrix ductility and an increase in coarse cavitation with increasing clay content. In this latter case, however, the zones of cavitation were clearly identified with breakdown of the clay aggregates or the interfaces between the clay and the matrix, underlining the important role of the clay-matrix interface for the properties of the styrenics, and by inference, those of the acrylics. To provide further insight into the role of the cellular structure of the styrenics, finite element (FE) simulations were used to investigate the distribution in hydrostatic stress in the nanocomposite films under finite deformations. These showed the principal stress concentrations to appear in the clay aggregates aligned in the direction of the applied deformation, while the hydrostatic stress in the matrix remained relatively uniform, suggesting that as long as the interface and clay aggregates remain stable, cavitation may initiate anywhere within the matrix, as in the unmodified polymer. The FE simulations were also used to model the elastic properties in the rubbery state, confirming the need to assume the presence of an immobilized matrix fraction in order to account for the observed Young's moduli. The results of these calculations were found to be consistent with those obtained from the foam-based micromechanical models, confirming the applicability of these latter, and suggesting the local anisotropy of the clay aggregates to play a limited role in the overall low strain elastic properties. The main outcome of this thesis has been the establishment of a general physical basis for predicting structure-mechanical property relationships in a new range of latex-based nanocomposite materials with exceptional properties, thanks to the possibility of incorporating very high clay loadings without compromising processability, and vast potential for fine tuning of stiffness and stiffness related properties. Moreover, the work has highlighted the important role of the "nano" effect, particularly at temperatures above the glass transition of the matrix, where matrix immobilization at the clay-matrix interface is clearly demonstrated to contribute to both thermal and mechanical properties. It is consequently expected to provide solid guidelines for the choice of morphological and materials parameters for the optimization of basic mechanical properties in such materials via controlled synthesis. Moreover, with further advances in synthesis and processing and hence in film quality, it should ultimately be possible to extend the basic physical model developed here to account for the structure dependence of other important applicative properties, such as permeability and fire resistance.

EPFL2010

Processing of fibre and porosity gradients in cellular thermoplastic composites

Manuel Bühler

EPFL2008

Highly filled polymer nanocomposite films derived from novel nanostructured latexes

Riccardo Ruggerone

EPFL2010