Epoxy nanocomposites – fracture and toughening mechanisms
Introduction
Epoxy resin systems are increasingly used as matrices in composite materials for a wide range of automotive and aerospace applications, and for shipbuilding or electronic devices. They serve as casting resins, adhesives, and as high performance coatings for tribological applications, such as slide bearings and calender roller covers. The mechanical property profile of epoxy matrices can be influenced, for example, by modifying the molecular architecture and structure, i.e. by increasing the crosslink density to generate high stiffness and strength [1]. Highly crosslinked epoxy matrices, however, often behave undesirably brittle because plastic deformation is constrained [2]. Moreover, the local stress concentrations may initiate cracks which lead to spontaneous failure. It is therefore a primary aim of many researchers to provide epoxy with higher toughness, but without significantly sacrificing other important characteristics such as thermo-mechanical properties and modulus, which are desired and required in many applications.
The commonly applied traditional toughening agents, however, often reduce other properties such as elastic modulus, strength, and thermo-mechanical properties, i.e. the glass transition temperature [3]. Such tougheners are, for example, liquid rubbers, spherical rubber particles [4], core shell particles [5], [6], glass beads [7], [8], microvoids [9], [10], [11], hyperbranched polymers [12], and combinations of these [13]. Modifiers less rigid than the polymer matrix may serve as excellent tougheners in matrices which show ductility to some degree. Rubber particles, for example, can induce the formation of microvoids which is subsequently accompanied by the activation of yielding processes due to the reduction of the local yield stress, i.e. the plastic resistance of the material. In this case, a substantial amount of energy is dissipated within the plastic zone near the crack tip. Rubber toughening, however, is to the expense in composite modulus. If the matrix is incapable of showing extensive ductility, toughening can be reached by introducing thermoplastic particles, which tend to act by promoting delocalised microcracking as well as crack bridging effects [14]. In this case, the spherical thermoplastic microparticles could avoid a decline in modulus of the composite. In rigid particle filled epoxies the toughening mechanism may comprise a combination of particle-matrix debonding, void formation around the particles, and subsequent yielding of the interparticle matrix ligaments [7], [15], [16], [17].
Natural composites give an example for materials with excellent property profile, since they combine high stiffness, strength and toughness, amongst other beneficial characteristics, e.g. ‘self-healing’ capabilities. As a structural characteristic, it is found that bones, teeth and sea shells take advantage of rigid nanoscale mineral platelets to reinforce a ‘soft’ polymeric collagen matrix [18], [19].
In order to develop technical nanocomposites inorganic nanoparticles were applied by many scientists to reinforce epoxy resins and other polymers. Much research has been performed on the incorporation of low and high aspect ratio nanofillers, and these have already demonstrated their capability to improve the toughness of polymers and other important properties such as wear resistance and electrical resistivity [20], [21], [22], [23], [24], [25], [26], [27], [28]. Still so far, high material costs, complex processes and limitations in production technology hamper the production and the application of these nanocomposites on a large industrial scale. Specifically, there are still difficulties to distribute individual nanoparticles homogeneously in the matrix.
A special characteristic of nanoparticles is their high specific surface area. Fig. 1 schematically shows the relationship between the particle size, the number and the total surface area in a composite with ideal microstructure, i.e. homogeneous particle distribution. Particularly, only 3 particles would be present if 3 vol.% microparticles (10 μm) are included. Nanoparticles (100 nm), instead, would increase the particle number to roughly 3,000,000 [29]. These provide a much larger interface area. In this case, a propagating crack front would interact with far more particles in the nanocomposite than in the microcomposite, and the fracture mechanical properties would change.
A high specific surface of fillers benefits the creation of a large portion of interface in the composite, provided, that the modifiers are perfectly dispersed in the matrix. Generally, the surface-to-volume ratio of particles increases with decreasing particle size. On the nanoscale the fraction of atoms localized at the surface is much higher than on the microscale. Therefore, the physical properties of particles of the same material can be different on the nanoscale, and they may even be dominated by quantum mechanical effects. It has been shown by experiment that the toughness and strength of structural ceramics rises strongly, if nanoparticles instead of microparticles are used as building blocks [30]. Moreover, the interactions between nanoparticles and the environment, e.g. the epoxy matrix, depend on the particles’ surface structure, geometry, and surface chemistry, and these have a great influence on the formation of the interface. How the interface will dominate the properties depends e.g. on the range of the influence of the interface, which may, however, be only a few nanometers in a highly crosslinked resin.
Nanoparticles constrain the matrix deformation less than microparticles, because they integrate better into the polymer microstructure as they approach nearly molecular dimensions. Depending on more or less strong interactions with the matrix, it can be expected that they influence deformation mechanisms in the polymer on the micro- or even the nanoscale. It is conceivable that nanoparticles could promote, for example, the formation of a large number of subcritical microcracks or microvoids and retard the collapse into critical cracks by coalescence.
Many properties, however, are still scale independent and the absolute number of particles per unit volume is then irrelevant.
In order to gain improvements of multiple properties, it is suggested that low or high aspect ratio fillers are incorporated into epoxy. These fillers should possess (1) a higher rigidity than the polymer to increase its stiffness, (2) a high specific surface, (3) a sufficient filler-matrix bonding to improve strength and to allow a controlled stress transfer from the matrix to the fillers, and (3) preferably small dimensions to reduce local stress concentrations and to generate high toughness and impact resistance. The reduction of the filler dimensions in brittle polymer composites is a promising pathway to improve the toughness, since the microstructural perfection of composites increases by minimizing the size of potential defects (e.g. inclusions, agglomerates). The minimization of defects serves as an important strategy to toughen brittle matrices such as structural ceramics [2], [30]; natural materials, in particular, make use of this strategy, and it is believed, that natural nanocomposites are less sensitive to flaws due to nanoscale fillers [31].
The aim of this study is to demonstrate the excellent properties of high performance epoxy nanocomposites by the characterization of the fracture mechanical properties, especially with regard to the static fracture toughness and the fatigue behaviour. Special attention is paid to the system epoxy/alumina. Furthermore, this work focuses to contribute to the understanding of reinforcing mechanisms induced by ceramic nanoparticles in epoxy resin.
Section snippets
Experimental
The materials in this study were a standard epoxy resin (DER331 by DOW) cured by a cycloaliphatic amine curing agent (HY 2954, Huntsman). The nanofillers were commercially available as powders. Aluminium oxide (Al2O3, type ‘Aeroxide Alu C’) with a primary particles size of about 13 nm, and without surface treatment, was provided by the Degussa AG. Titanium dioxide (TiO2, type ‘Kronos 2310’) possesses diameters in the sub-micron scale between 200 and 500 nm. These TiO2 particles were surface
Microstructure of nanocomposites
Transmission electron microscopy (TEM) is a straightforward technique to visualize the dispersion quality of nanoparticles within epoxy. TEM pictures of EP/Al2O3 nanocomposites, as manufactured by mechanical dispersion techniques, are displayed in Fig. 3. A homogeneous distribution of particles is visible, although some small agglomerates remain present in the matrix. The size of these agglomerates is in the range between 30 and 50 nm. Some of them are not spherical and possess
Acknowledgement
The authors gratefully acknowledge the support of the German Federal Ministry of Education and Research (BMBF) (Project No. 03X5500).
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