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Poly (ether ether ketone) (PEEK) has a high continuous service temperature, excellent mechanical properties, and good solvent and abrasion resistance, which can be further improved through the addition of carbon nanotubes (CNTs). CNT-PEEK nanocomposites are promising matrices for continuous carbon fibre composites; powder processing can mitigate the high melt viscosities in these systems. In this study, model single fibre (hierarchical) composites were produced by embedding sized and desized carbon fibres in nanocomposite CNTPEEK powders followed by single fibre pull-out tests to assess interfacial characteristics. Carbon fibre-PEEK interfacial shear strength is typically 40–45 MPa. Increasing CNT loadings increased fibre-matrix interfacial shear strength linearly up to ~70 MPa at 5.0 wt%, which was attributed to the CNT-based mechanical modification of the PEEK matrix. Apparent interfacial shear strength was inversely correlated with the embedded fibre length irrespective of carbon fibre sizing or CNT loading, indicating brittle fracture of the fibre-matrix interface.
Pulled out carbon fibres were still coated with the matrix, which indicated strong adhesion at the interface in all samples, likely related to a transcrystalline region. Adhesion was, however, negatively affected by the presence of epoxy sizings. Frictional shear strength was independent of embedded fibre length and CNT content for all samples.
In this work, a closed loop recycling process is investigated, which allows polymerised bulk thermoplastic matrix (Elium 150) from production waste (also referred to as recyclate) to be reused as additive in composite manufacturing by vacuum assisted resin infusion (VARI) of virgin Elium 150 monomer. It is shown that this process can save up to 7.5 wt% of virgin material usage in each processing cycle. At the same time, the thermal stability and stiffness of the composite increases with the proportion of recyclate introduced. Contemporarily, the shear and bending properties have also been observed to improve. Gel permeation chromatography (GPC) showed that the changes observed are due to an increase in molecular weight with the recyclate content. In particular, a correlation between the molecular weight and the shear properties of the composite was discovered using single fibre push-out tests.
Assessing the structural integrity of carbon-fibre sandwich panels in fire: Bench-scale approach
(2019)
The fire resistance of lightweight sandwich panels (SW) with carbon fibre/epoxy skins and a poly(methacryl imide) (PMI) foam core is investigated in compression under direct application of a severe flame (heat flux=200 kW m−2). A bench-scale test procedure was used, with the sample held vertically. The epoxy decomposition temperature was quickly exceeded, with rapid flash-over and progressive core softening and decomposition.
There is a change in failure mode depending on whether the load is greater or less than 50% of the unexposed failure load, or in other words if one or two skins carry the load. At high loads, failure involved both skins with a single clear linear separation across each face. There is an inflection in the failure time relationship in the ∼50% load region, corresponding to the time taken for heat to be transmitted to the rear face, along with a change in the rear skin failure mode from separation to the formation of a plastic hinge. The integrity of the carbon front face, even with the resin burnt out, and the low thermal diffusivity of the core, both play key roles in prolonging rear face integrity, something to be borne in mind for future panel design. Intumescent coatings prolong the period before failure occurs. The ratio of times to failure with and without protection is proposed as a measure of their effectiveness. Apart from insulation properties, their adhesion and stability under severe fire impact play a key role.
Nebulizing of polymer solutions, in a high-voltage field under atmospheric conditions by electrospray ionization (ESI), is a comfortable way to deposit ultra-thin layers of polar or ionic polymers onto any conductive substrate materials. The substrate is grounded and the polymer solution is sprayed through a powered capillary. The formed charged droplets shrink by solvent evaporation during their way to the grounded substrate, the charges close ranks and the droplets collapse consecutively by charge repulsion, thus forming finally charged single macromolecules. After their discharging at the grounded substrate, an ultrathin ‘quasi-monomolecular’ polymer layer is formed. It could be shown by imaging of scratches through the polymer layer by atomic force microscopy that the deposited polymer layers are dense at a thickness of about 10 nm. Carbon fibre bundles were coated with poly (allylamine) (PAAm) or poly(acrylic acid) (PAA) as potential adhesion-promoting layers in fibre–polymer composites. The polymer deposition is self-inhibiting after formation of a continuous coverage of about 200 nm for PAAm and 30 nm for PAA as result of surface charging. Continuous deposition onto such isolating layers or polymers without charging can be achieved by using current of alternating polarity. The film formation is self-healing because of the electrophoretic effect, i.e. the ion discharging occurs preferentially at noncoated areas. This electrophoretic effect of ESI was demonstrated by completely enwrapping all the carbon fibres of the roving within a distance of about 100 μm far from its outside and also at the backside of the fibre bundle with about 80% of the topside coverage, as measured by X-ray photoelectron spectroscopy and visualized using scanning electron microscopy.
Ceramic tapes with different chemical compositions were produced by tape casting technology. Unlike the traditional solvent-based slurries, we have used eco-friendly water based formulations. These thin coatings were afterwards used as top layer on 6-layered carbon fibre epoxy composites as intended fire protective coatings. Tape C, containing cellulose fibres, has demonstrated to be the most effective coating regarding the fire reaction behaviour. These coatings are attractive in the sense that: Their composition can be more or less tailored, there are easily impregnated with epoxy resin, and they are extremely flexible, and can therefore be used in a wide range of applications.
Multifunctional composites which can fulfil more than one role within a system have attracted considerable interest. This work focusses on structural supercapacitors which simultaneously carry mechanical load whilst storing/delivering electrical energy. Critical mechanical properties (in-plane shear and in-plane compression performance) of two monofunctional and four multifunctional materials were characterised, which gave an insight into the relationships between these properties, the microstructures and fracture processes. The reinforcements included baseline T300 fabric, which was then either grafted or sized with carbon nanotubes, whilst the baseline matrix was MTM57, which was blended with ionic liquid and lithium salt (two concentrations) to imbue multifunctionality. The resulting composites exhibited a high degree of matrix heterogeneity, with the ionic liquid phase preferentially forming at the fibres, resulting in poor matrix-dominated properties. However, fibre-dominated properties were not depressed. Thus, it was demonstrated that these materials can now offer weight savings over conventional monofunctional systems when under modest loading.
Thin layers of textile reinforced concrete (TRC) can be used as strengthening for reinforced concrete (RC) slabs or beams to increase their foad-bearing capacity. The concrete matrix consists of a fine-grained concrete with a thickness of 8 to 15 mm. Information on the behaviour of strengthened structures subjected to fire is of particular inlerest for practical applications of this innovative construction method. A total of 7 RC slabs with Steel reinforcement were produced: 2 slabs were used as reference and 5 slabs were strengthened with three layers of textile reinforcement. Ultimate capacity of reference slabs and particular TRC strengthened slabs was determined from statie load results fom 4-point-bending tests.
Fire tests on these slabs were carried out with utilization fectors using a Standard temperature curve (ISO-834-1) for fire testing. Test loads varied between 0 % and 65 % ofuMmate load.
To detect temperatures during fire exposure thermocouples were arranged throughout the cross-section of the slabs. Depending on the utilization the slabs were exposed to fire for 30 minutes or for 60 minutes. In some cases bending fäilure occurred, in other cases the slabs survived. None of the specimens exhibited any visual damage or concrete spalling during fire exposure. Results of deformation measurements and residual strength measurements are presented. Even though mechanisms are not fifily understood, it is important to discuss the presumable reasons for the surprisingiy positive test results. It is presumed tbat the key mechanisms contributing to the outstanding fire resistance capability presented are: superior crack control of the TRC and load redistribution between textile and Steel reinforcement as well as the primary load transfer directly into the slabs.
A simple method of creating defined PMMA and poly (MMA-co-Cz) electrocoatings on carbon fibres is described. The electrodeposition of poly methylmethacrylate (PMMA) onto unsized, unmodified carbon fibres was performed by simple constant current electrolyses of methylmethacrylate (MMA) monomer in dimethylformamide (DMF) solutions and the pure liquid monomer using sodium nitrate and lithium perchlorate as supporting electrolytes. The presence of polymeric coatings successfully attached to the carbon fibres was verified by scanning electron microscopy and photoelectron spectroscopy (XPS).
Performing the electrolysis in dilute MMA in DMF solutions ([MMA] < 5 M) results in the deposition of powder-like polymer on the carbon fibre electrodes. Increasing the MMA concentration in the DMF solution results in a homogeneous PMMA coating of the carbon fibres. The degree of grafting or coating increases with increasing MMA concentration, except when pure MMA is used without solvent.
The adhesive strength between the electrocoated carbon fibres and a PMMA matrix was determined using the single fibre pull-out test. It was found that the interfacial fracture behaviour of all carbon fibre/PMMA model composites is rather brittle. The adhesion strength between the unmodified carbon fibres and the PMMA matrix was equal to the cohesive strength of the polymer matrix itself. Nevertheless, the electrodeposition of thin and homogeneous PMMA coatings resulted in much improved adhesion strengths.