Silk fibers combine good stiffness and strength with a very high strain to failure and are as such highly promising to realize composites with high impact resistance. It is shown that to realize this potential it is quite beneficial to employ matrix materials of high strain to failure, particularly thermoplastic matrices. High impact resistance is thus achieved, well above the values for the pure matrices. Below the glass-transition temperature of the thermoplastic matrix, the impact energy absorption decreases. The adhesion between fiber and matrix also plays a significant role; lower adhesion typically increases the low-velocity penetration impact resistance, due to the spread of damage. Finally, the fiber architecture is pivotal; when a woven fabric is used which is unbalanced in strength, the impact resistance reduces in correspondence with the weakest material direction. A quasi-isotropic layup has a lower capacity for deformation than a balanced woven configuration which likely explains the observed lower penetration impact resistance.

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The combination of ductile Bombyx Mori silk fibre with ductile thermoplastic matrices, combined with the use of a woven fibre architecture, leads to ductile composites with quasi-isotropic strength. Bending and tensile properties were determined for (0,90)_s silk twill weave composites, with different thermoplastic polymers and as reference epoxy resin. Thermoplastic (0,90)_s woven silk fibre composites are isotropic in strength (about 135 MPa at 50% fibre volume fraction) due to the high elongation to failure of both the silk fibre and the thermoplastic matrix allowing extensive shearing of the silk fabric when loaded in the 45° direction. A good quality interface has a clear positive influence on the mechanical properties especially on the bending yield strength.

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Natural materials such as wood exhibit high mechanical properties through cellulose structured at multiple length scales and embedded in a matrix of similar chemical structure. These hierarchical materials have inspired the design of lightweight composites composed of naturally occurring polymers. However, the close proximity of melt and decomposition temperature remain a challenge. In this work, cellulose propionate (CP) is modified to reduce its glass transition temperature and melt viscosity, allowing its use as a matrix in a natural fibre-reinforced composite. Through better impregnation, the modified CP matrix composites showed an increase in stiffness and strength of ∼10% and 20%, respectively, in comparison to unmodified CP matrix composites. The impact properties also increased by up to 28%, showing that modified CP is a credible matrix for realising sustainable all-cellulose natural fibre composites with high stiffness, strength and toughness.

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To evaluate compatibility between a substrate and a thermoplastic polymer, the established methodology is to estimate their surface composition in terms of surface energy components, utilizing the results of contact angle measurements of probe liquids onto substrate and polymer surfaces at room temperature. Using this methodology, polymer surfaces are studied in solid state, however, during spreading of polymers on a substrate, polymers are in molten state and at high temperature, having different surface energies and more complex polymer/substrate interactions due to polymer chain mobility. This paper presents a model study with practical relevance to predict polymer/substrate compatibility including contact angle measurements at high temperature directly performed between molten thermoplastics; polypropylene (PP), polyvinylidene fluoride (PVDF) and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and plates. The values of total surface energy of thermoplastics at high temperature (260 °C) are down to 57% of that measured at room temperature, which has a strong influence on the wetting prediction. Surface energies of both the polymer and the substrate were found not to be the only factor controlling the wetting behaviour of molten polymers and the level of adhesion with the substrate, but also some intrinsic characteristics of the polymer melt play a role. We also observed that the wetting behaviour of molten MAPP is affected by the maleic anhydride (MA) content, demonstrating dramatically different results to room temperature measurements, which is suspected to be due to the formation of covalent bonds of MA groups with the glass surface enhancing the interface strength beyond the shear strength of MAPP.@en

Direct contact angle measurements were performed between different molten thermoplastics, polypropylene (PP), polyvinylidene fluoride (PVDF), and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and smooth glass plates. The matrices were selected as model systems for the investigation of the fibre–matrix interphase, based on the difference in surface energies between PP and PVDF (physical interactions) and the effect of chemical bonding between PP and MAPP. In this way, physical and chemical adhesion were studied independently. The mechanical strength of the interfaces was then assessed by single fibre pull-out tests. The interfacial strength and the wettability of molten thermoplastics correspond well to the predictions based on the calculation of the theoretical work of adhesion between the matrices and the fibre surfaces for PP and PVDF; however, the wetting behaviour of molten MAPP is affected by the MA content if compared with the surface energy analysis made at room temperature. The hypothesis is that the wetting behaviour of molten MAPP is mainly related to chemical interaction of MA groups with the glass surface rather than a variation in surface energy.

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Natural fibres (NF) have shown to be a suitable alternative to glass fibres as reinforcing material in polymer composites since they offer similar specific strength and stiffness. Thus, those composites may be readily used in the automotive, sport and leisure. However, most of today's natural fibre applications are based on discontinuous fibre architectures, underrunning the performance of continuous synthetic fibre composites. Flax fibres show a great potential to be used as continuous reinforcing fibres in thermoplastic matrix composites. The use of high performance engineering polymers and the application of preceding plasma-based fibre surface treatments may further enhance the mechanical properties, making flax fibre thermoplastic composites an environmentally friendly alternative. We investigated the mechanical behaviour of pure flax fibres, introduced an engineering polymer as matrix system, compared this material to commonly used matrices such as epoxy or polypropylene and studied the effect of two different plasma treatments on the mechanical performance of natural fibre composites (NFC). The influence of the plasma methods was found through composite failure testing. Results have shown, that composites with plasma treated fibres and engineering polymers along with a high fibre volume content offer a great potential as environmentally sustainable substitutes for synthetic composites in many applications.@en