Bernd Wetzel

Epoxy nanocomposites with high mechanical and tribological performance

Abstract Small ceramic particles are known to enhance the mechanical and tribological properties of polymers. Introduced into an epoxy resin, the filler morphology, size, particle amount and the dispersion homogeneity influence extensively the composites performance. In the present study, various amounts of micro- and nano-scale particles (calcium silicate CaSiO3, 4–15 μm, alumina Al2O3, 13 nm) were systematically introduced into an epoxy polymer matrix for reinforcement purposes. The influence of these particles on the impact energy, flexural strength, dynamic mechanical thermal properties and block-on-ring wear behavior was investigated. If the nanoparticles were incorporated only, they yield an effective improvement of the epoxy resin at a nanoparticle content of already 1–2 vol.% Al2O3. Choosing the nanocomposite with the highest performance as a matrix, conventional CaSiO3 microparticles were further added in order to achieve additional enhancements in the mechanical properties. In fact, synergistic effects were found in the form of a further increase in wear resistance and stiffness. Several reasons to explain these effects in terms of reinforcing mechanisms were discussed.

Effect of particle surface treatment on the tribological performance of epoxy based nanocomposites

To overcome the disadvantages generated by the loosened nanoparticle agglomerates dispersed in polymer composites, an irradiation grafting method was applied to modify nanosilica by covalently bonding polyacrylamide (PAAM) onto the particles. When the grafted nanosilica was added to epoxy, the curing kinetics of the matrix was accelerated. Moreover, the grafting PAAM can take part in the curing of epoxy so that chemical bonding was established between the nanometer fillers and the matrix. Sliding wear tests of the materials demonstrated that the frictional coefficient and the specific wear rate of nanosilica/epoxy composites are lower than those of the unfilled epoxy. With a rise in nominal load, both frictional coefficient and wear rate of the composites decrease, suggesting a wear mechanism different from that involved in wearing of epoxy. Grafted nanosilica reinforced composites have the lowest frictional property and the highest wear resistance of the examined composites. Compared with the cases of microsized silica and untreated nanosilica, the employment of grafted nanosilica provided the composites with much higher tribological performance enhancement efficiency.

Microstructure and tribological behavior of polymeric nanocomposites

Nanocomposites represent a new prospective branch in the huge field of polymer materials science and technology. It has been shown that an overall enhancement of properties of polymers can be achieved under certain conditions by the addition of nanoparticles. To examine the influence of microstructure on the tribological performance of nanocomposites, different ways of compounding were used in this study. It was found that the friction and wear behavior of polymeric nanocomposites under sliding environment was rather sensitive to the dispersion states of the nanoparticles. When the microstructural homogeneity of the nanocomposites was improved, their wear resistance could be increased significantly. The present work demonstrates the importance of TiO2‐nanoparticles dispersion in an epoxy resin matrix, on the materials’ tribological properties, when sliding against a smooth steel counterpart.

Improvement of tribological performance of epoxy by the addition of irradiation grafted nano-inorganic particles

To develop wear resistant nanocomposite coating materials, the authors of the present work treated nanosilica first by introducing a certain amount of grafting polymers onto the particles in terms of an irradiation technique. Throught irradiation grafting, the nanoparticle agglomerates turn into a nanocomposite microstructure (comprised of the nanoparticles and the grafted, homopolymerized secondary polymer), which in turn built up a strong interfacial interaction with the surrounding epoxy matrix through chain entanglement and chemical bonding during the subsequent mixing and consolidation. The experimental results indicated that the addition of the grafted nanosilica into epoxy significantly reduced wear rate and frictional coefficient of the matrix at low filler loading. Compared with the cases of microsized silica and untreated nonosilica, the employment of grafted nanosilica provided composites with much higher tribological performance enhancement efficiency. Unlike the approaches for manufacturing of other types of nanocomposites, the current method is characterized by many advantages, suchs as simple, low cost, easy to be controlled, and broader applicability.

Graft polymerization onto inorganic nanoparticles and its effect on tribological performance improvement of polymer composites

Abstract To overcome the disadvantages generated by the loosened nanoparticle agglomerates dispersed in polymer composites, a chemical grafting method was applied to modify nano-alumina, silicon carbide and silicon nitride through covalently introducing polyacrylamide (PAAM) onto the particles. Sliding wear tests demonstrated that the frictional coefficient and specific wear rate of the nanoparticles/epoxy composites are lower than those of unfilled epoxy. Grafted nanoparticles reinforced composites have the lowest frictional property and the highest wear resistance due to the strengthening of the nanoparticle agglomerates and the enhancement of filler/matrix interfacial interaction resulting from the grafting polymers. Comparatively, graft treatment of nanoparticles is more beneficial to the improvement of the tribological features of the composites than the silane treatment that is used conventionally.

Thermo-molded self-healing thermoplastics containing multilayer microreactors

Here in this work we propose a thermally molded self-healing thermoplastic polymer containing a multilayer microcapsule-type microreactor. The latter consists of an isolated monomer and catalyst system. As the first proof-of-concept composites, commercial plastics polystyrene (PS) filled with glycidyl methacrylate (GMA)-loaded microreactors are reported. The specially designed structure of the microreactors enables them to be robust enough to survive the thermal processing already widely used in the plastics industry, like melt mixing under shear and compression molding. Upon damage of the composites and microreactors, the GMA monomer is released to the cracks and its atom transfer radical polymerization is triggered when it passes by the catalyst layer. The polymerized GMA serves as a macromolecular adhesive, which not only fills up the interstitial gap of the cracks but also is anchored to the sub-surface of the matrix forming mechanical interlocking, offering satisfactory healing efficiency at room temperature. By changing the species of healing monomer, catalyst and wall substances, the microreactors can meet the versatile requirements of different polymers, so that the approach is provided with broad applicability.

Finite Element Simulation of the Fiber–Matrix Debonding in Polymer Composites Produced by a Sliding Indentor: Part I – Normally Oriented Fibers:

To study the contact and debonding behaviors between a CF–PEEK fiber-reinforced polymer composite specimen and a sliding diamond indentor, finite element macro- and micro-models have been developed. Around each fiber, interface elements were introduced in order to detect the tension-type and also the shear-type debondings for different cases. If initial or final debonding has occurred, a “control algorithm” checked the limit strain conditions for the interface elements and changed the material properties according to the actual debonding condition. As a final result, it can be concluded, that the dominant debonding under compression is due to the shear loading conditions.

Erosive and sliding wear of polybenzimidazole at elevated temperatures

Polybenzimidazole (PBI) is the most sought polymer for high thermal stability, mechanical strength and retention at elevated temperatures. However, its potential for tribological applications, especially at high temperatures and under various wear modes, is not yet explored in depth. In this work, commercially available PBI was investigated under sliding wear against steel at elevated temperatures and different pressures. A relation between the linear wear rate and the test temperature could be shown. But the wear mechanisms do not differ very much within the temperature range tested. In addition, the solid particle erosion resistance of PBI under different impact angles and surface finishes was investigated at ambient and high temperature. A higher erosion rate took place when the temperature was increased. Overall, PBI showed a semi-brittle erosive failure (αmax at 45°) behaviour. Scanning electron microscopy was used to understand the wear mechanisms in more detail, and white light profilometry allowed to get information about the topography of the fresh and eroded surfaces.

Effects of graphene and CNT on mechanical, thermal, electrical and corrosion properties of vinylester based nanocomposites

Abstract Vinylester nanocomposites with graphene and carbon nanotubes were investigated regarding thermal and viscoelastic behaviour, flexural properties, hardness, scratch resistance and electrical conductivity. The latter showed a percolation threshold for the graphene nanocomposites at ∼2 wt-%, whereas the nanotube system exhibited this transition at 0·03 wt-%. Investigations (SEM) by charge contrast imaging demonstrated a high degree of dispersion of both fillers, with the formation of a continuous percolation network. Quantitative tests regarding the corrosion resistance revealed that the conductive hybrid composites, applied as a coating to a metal substrate, exhibited a better performance than the neat vinylester.

Finite Element Simulation of the Fiber– Matrix Debonding in Polymer Composites Produced by a Sliding Indentor: Part II – Parallel and Anti-Parallel Fiber Orientation:

A debonding simulation technique, based on a series of nonlinear FE evaluations, has been developed to study the contact behavior and the debonding process of unidirectional fiber-reinforced polymer composites subjected to a sliding diamond indentor. In Part I, the normal fiber orientation was studied, while in Part II, the parallel and anti-parallel fiber orientations relative to the sliding direction are analyzed. The technique developed can consider both the shear and the tension type debonding events, and also the effect of friction causing limited debonding. The results show a dominant limited shear type debonding under compression for both fiber orientations.