Alois K. Schlarb

Ultrasonic dispersion of inorganic nanoparticles in epoxy resin

The incorporation of nanoscale fillers into a polymer can lead to a considerable improvement of mechanical properties, i.e. stiffness and toughness of a material can be enhanced simultaneously by the insertion of nanofillers. Thereby, the crucial difference between conventional microscale fillers and nanofillers is the high specific surface of the latter. In order that this surface can interact with the matrix material a good dispersion, i.e. a good separation and a homogeneous distribution of the nanoparticles into the polymer, is required. In the present study ultrasonic waves generated by an ultrasonic horn were used to disperse titanium dioxide nanoparticles into epoxy resin. The process parameters, e.g. the ultrasonic amplitude, the dispersion time and the materials volume, were varied systematically with the aim of achieving an optimum dispersion process. A dispersion model for bead mills was adapted to the ultrasonic process and compared to a second dispersion model in order to find an adequate mathematical expression to correlate the ultrasonic process parameters to the particle sizes in the material and to allow predictions for further experiments.

Preparation of TiO2/epoxy nanocomposites by ultrasonic dispersion and their structure property relationship.

By the insertion of nanoparticles into a polymer matrix a considerable improvement of mechanical properties can be achieved. Therefore, a homogeneous distribution of fillers within the matrix is required. In the present paper the dispersion of TiO(2)-nanoparticles in a DGEBA (diglycidyl ether of bisphenol A) epoxy resin by means of an ultrasonic horn was studied. The systematic examination of process parameters of a previous study was completed in order to determine the optimum processing window leading to a good dispersion result without degrading the molecular structure of the epoxy resin. Therefore, particle sizes were examined using a dynamic light scattering device, and the effect of the ultrasonic treatment on the resin was surveyed by FT-IR spectroscopy (Fourier transform infrared spectroscopy). Furthermore, the mechanical performance of the nanocomposites was examined for various contents of TiO(2)-nanoparticles to show that the materials prepared by ultrasonic dispersion show an improved propertys profile. In order to understand the reinforcing mechanisms of nanoparticles in the polymer matrix providing improved mechanical properties, scanning electron microscope (SEM) pictures of the fracture surfaces of the samples were carried out, which revealed that nanocomposites show a significantly rougher surface than the neat epoxy resin. This indicates a change in the fracture mechanisms.

Rolling wear of EPDM and SBR rubbers as a function of carbon black contents: correlation with microhardness

The rolling friction and wear of ethylene/propylene/diene (EPDM) and styrene/butadiene rubbers (SBR) with different carbon black (CB) contents were studied against steel in orbital rolling ball (steel)-on-plate (rubber) test rig (Orbital-RBOP) and oscillating rolling ball (steel)-on-plate (rubber) set-up (Oscillating-RBOP). The universal hardness (H), coefficient of friction (COF), and specific wear rate (Ws) of EPDM and SBR were determined. Incorporation of CB increases the universal hardness and the COF (the latter marginally) and decreases the specific wear rate for both EPDM and SBR. The wear mechanisms were concluded by inspecting the worn surfaces in scanning electron microscope and discussed as a function of CB modification. An inverse relationship between the specific wear rate and universal hardness was proposed in form of Ws = kH−n, where k and n are constants for a given rubber and testing condition.

Modelling of Mechanical Properties of Nanoparticle-Filled Polyethylene

A continuum-mechanics approach has been applied to model the mechanical properties of high density polyethylene (HDPE) filled with differently coated SiO2 nanoparticles. The tensile yield stress has been predicted with a finite-element model (FE-model) taking into account the microstructural features, e.g. matrix, filler content and the interphase formed around the filler. A good agreement between the experimental and modelled data has been found. Additionally, the FE-model has been compared with a semi-empirical model and an analytical model with similar input parameters to predict the composite property. It was found that all models are congruent with respect to solution space.

Mechanisms of Friction and Wear Reduction by Carbon Fiber Reinforcement of PEEK

Carbon fibers are widely used as reinforcements in poly-ether-ether-ketone (PEEK). In recent years, these materials have also been used for tribological applications. For further optimization of these tribo-materials, the contribution and action mechanisms of carbon fiber reinforcements to the tribological performance of PEEK composites need to be understood. Toward this goal, we have studied carbon fibers in a PEEK composite by scratching experiments using Berkovich and conical indenters and friction imaging using contact atomic force microscopy. For comparison, scratching was extended into the PEEK matrix surrounding the carbon fibers. It is found that shearing dominates the friction and wear behavior of carbon fibers alone, while both shearing and plowing contribute to the overall friction of PEEK composites. There is no local variation in friction across a carbon fiber surface. The wear reduction by carbon fibers originates from their effective load-bearing capability. For the first time, fatigue of individual carbon fibers is revealed, as well as the dependence of interfacial debonding or delamination on the contact configuration between fibers and scratching asperities.

Resistance to time-dependent deformation of nanoparticle/polymer composites

Dimensional stability and structural reliability are major concerns for thermoplastics due to their nature of high mobility of polymer chains. Here the authors report the creep experiments on polypropylene filled with 1vol% TiO2 nanoparticles with diameter of 21 or 300nm at different load levels and temperatures, which show that the dimensional stability of the nanocomposites is significantly improved, especially by loading small sized nanoparticles. The creep strain and creep rate of nanoparticle-filled polypropylene are reduced by 46% and 80%, respectively, compared to those of the neat matrix. Additionally, creep lifetime is extended by 330% due to the addition of small nanoparticles.

Mechanical and thermal behaviours of polyetheretherketone-based multi-scale composites:

In this study, polyetheretherketone composites were compounded using a two-screw extruder followed by injection moulding. The effects of multi-fillers on the mechanical properties and crystallization performances were investigated. Differential scanning calorimetry results indicate that the addition of fillers slightly increases the crystallization temperature and crystallinity. Compared to neat polyetheretherketone, the incorporation of inorganic filler leads to a significant improvement in matrix hardness, matrix stiffness and a slight increase in tensile strength. However, the material ductility, the impact strength and the fracture toughness of polyetheretherketone composites decrease. Fractography analyses show that the addition of fillers restraints the ductile deformation of polymers, which is responsible for the reduction of material ductility, impact strength as well as fracture toughness of polyetheretherketone composites.

Analysing dielectric interphases in composites containing nano- and micro-particles

We have investigated a molecular relaxation process in a solid polymer filled with dispersed magnetite particles (Fe3O4 in epoxy resin). In particular, we compare systems containing nano-particles with diameters between 20 and 30 nm and micro-particles with diameters between 0.5 and 5 µm. Temperature-dependent broadband dielectric spectroscopy in a frequency range between 50 Hz and 1 GHz reveals that the presence of nano- or micro-particles does not affect the molecular dynamics, i.e. frequency, shape and thermal activation of the relaxation process. However, there is a marked difference in the polymers relaxation strength, reflecting both the polarizability and the number of relaxing units. This quantity is evaluated from the measured effective data using the spectral representation, i.e. in spite of the complex microstructure we are able to separate unambiguously the contribution of the polarized conductive particles. While in the micro-composites the polymer matrix behaves bulk-like, its relaxation strength increases in the nano-composites, the deviation from the bulk value being proportional to the volume fraction of particles. We discuss the results in terms of interphases of thickness δ around particles and agglomerates, the volume fraction of which increases with increasing particle concentration and decreasing particle size.

Creep resistance of thermoplastic nanocomposites

Relatively poor creep resistance is considered as a deficiency of thermoplastic materials in general. In order to comprehensively and deeply study the effect of nanoparticles incorporated into a polyamide matrix a systematic investigation on various kinds of nanofillers under differed temperatures and stress levels have been performed. It was found that the creep resistance of polyamide 66 can be significantly enhanced due to the incorporation of nanofillers and the behavior of the composites can be easily described by the model of Findley.

Novel Experiments Reveal Scratching and Transfer Film Mechanisms in the Sliding of the PEEK/Steel Tribosystem

In order to reveal fundamental tribological mechanisms in polymer/steel sliding pairs, the pin-on-flat configuration of classical macroscopic tribotests was transferred into a high-resolution tribometer designed for scratch tests. Experiments were performed with a polyetheretherketone (PEEK) pin sliding on a steel disk in straight unidirectional movement mode. The surface morphology was monitored by interrupting the tests every 10,000 sliding strokes. The evolving surface morphology of PEEK was correlated with the transfer layer formed on steel counter surface. Scratching grooves in the PEEK surface were induced by asperities at the counter steel surface covered with transfer layers. Transfer layers were composed of lumpy polymer material accompanied by fine wear debris in areas of lower roughness. These smooth areas limit the penetration of large asperities and distinguish the scratching mechanism in macroscopic sliding from typical single-asperity scratching tests. The results reveal the mechanisms leading to inhomogeneity in the transfer layers as consequence of the asperity distribution.