Filling the gaps - advanced composite benefits
Thanks to their strength and lightweight properties, composites are replacing traditional metals and plastics in many fields. Dr Andrew Curtis, Dr Will Palin and Dr Adrian Shortall look at how these materials offer improved characteristics through nanochemistry advances and modern testing.
Advanced composite materials have entered many aspects of our lives, in particular areas such as dental restorative materials. These composites must resist a complex array of mechanical loads, aqueous environments, pH variations and temperature fluctuations, while being repairable and mimicking the appearance of the natural dentition.
The use of mechanically superior dental silver amalgams is declining, due to low aesthetic quality along with the need for invasive macro-mechanical cavity preparation, environmental concerns and public perception of the health issues surrounding mercury content. Meanwhile, an extensive array of resin composite materials has been developed in the search for a viable alternative.
Historically, the clinical longevity of these composites was not comparable with that of amalgams. However, the ongoing development of new composite materials means the longevity of modern composites is equivalent to or better than the amalgam. A greater level of understanding of the techniques involved in placing composite restorations has been attained, along with improved training of dental students.
The methacrylate chemistry that dental composites is based upon has changed little since its introduction in the early 1960s, while the size of the reinforcing filler particles has been gradually reduced to the point where the filler size has been described to be within the nanoscale. Although the description ‘nano’ may be seen as commercially motivated, the drive behind the reduction in particle size has been to increase filler packing and loading. This improves physical and mechanical properties, such as polish retention and resistance to failure.
Although so-called nanohybrid or nanofilled composites have demonstrated a degree of clinical and experimental success, controversy remains as to their specific benefit compared with existing, conventionally filled micro-hybrid systems.
Nanofills and hybrid composites
One specific composite development focused on a resin reinforced with a nanocluster phase. The nanoclusters are agglomerations of zirconium dioxide and silicon dioxide particles (less than 20nm) that have been calcined and silanated to create a micron-sized particle. This particle is then treated with a silane coupling agent to promote interfacial bonding when the clusters are incorporated into the methacrylate resin matrix. The resulting nanoclusters are shown in the micrographs.
To provide a comparison with conventional composites, two further materials were tested. The first is a microhybrid consisting of spherical micronsized zirconium dioxide and silicon dioxide particles incorporated into a resin matrix identical to the nanocluster composite displayed in the Cryo-SEM. The second is a nanohybrid consisting of micron-sized irregular barium-alumina borosilicate particles and submicron silicon dioxide incorporated in the matrix. As with all modern dental composites, polymerisation of the resin phase was activated by high-intensity visible light, initiating a rapid free-radical addition reaction.
Micromanipulation of nanoclusters and filler particles
The failure mechanism of individual filler particles is seldom tested. To remedy this, a micromanipulation technique (essentially, a micro-compression test) was used to determine the failure mechanism and properties of individual filler particles (nanocluster, micro-spheroidal and irregular) separated from the polymeric resin matrix.
Testing revealed that the nanoclusters underwent up to four failure events, while the spheroidal and irregular particles tested underwent either no failures or a single fracture, which occurred at a statistically lower strength than the nanoclusters. This was attributed to the ability of the nanocluster to collapse into pre-existing cluster porosities, as well as progressive fragmentation of the main cluster structure.
This novel failure mechanism suggests that incorporation of the nanoclusters into the resin matrix as part of the complete system has the potential to produce unique mechanical properties. This is due to deformation of the particle and subsequent enhanced resistance to crack propagation, thereby improving filling longevity.
Right, from top: Cryo-SEM fracture surface images of nanoclusters (79.0wt%). Cryo-SEM fracture surface images of spheroidal nanoclusters (84.5wt%). Cryo-SEM fracture surface image of nanocluster composite following exacerbated degradation in solvent (NaOH).
Resin composite fatigue
As resin composites are exposed to repeated load and fatigue during their service life, the response of the nanocluster (and two conventional composites) to realistic clinical loads was tested. While conventional materials underwent the expected gradual reduction of mechanical properties, the nanocluster composite was unaffected and in some cases presented improved properties. This was consistent with the micromanipulation results, which already suggested the nanoclusters were likely to provide a unique reinforcement.
It appeared that while the planar faces or sharp surface roughness of irregular fillers did not effectively deflect or dissipate the crack-energy – something at which the spheroidal fillers were more effective – the nanoclusters did provide a more damage-tolerant system. This was thought to be a result of the nanoclusters’ ability to deform and dissipate the accumulated fatigue loading stresses. Essentially, the nanoclusters acted as shock absorbers within the resin system.
Resin composite hydrolysis and chemolysis
To this point, mechanical property studies of the nanocluster material showed promise compared with the conventional systems. However, these results had been obtained under dry conditions or after short, 24-hour periods immersed in an aqueous environment. Therefore the nanocluster and conventional composites were immersed in an aqueous solution for up to 18 months and in solvents for up to two weeks, in order to exacerbate degradation. Fourier transform infrared spectroscopy and near-infrared spectroscopy were used to determine the extent of water sorption into the material, and to correlate this with the subsequent mechanical properties.
As expected, the mechanical properties of all materials were substantially reduced following immersion, due to hydrolytic degradation of the resin matrices and interfacial silane bonds. Comparing the polymer matrix of the nanocluster with spheroidal particle-reinforced composites revealed that the nanoclusters were particularly prone to water uptake and hydrolysis. This was attributed to the higher surface area to volume ratio of the constituent nanoparticles, resulting in greater silane wetting and, therefore, degradation and reduced cluster integrity. Likewise, chemolysis of the matrix and silane occurred as a result of solvent immersion, as seen in the Cryo-SEM fracture surface images. It was identified that a constituent monomer of the nanocluster resin was particularly prone to chemolysis.
Overall, while long-term hydrolytic stability may be limited, nanocluster reinforcement has the potential to provide enhanced damage tolerance and, therefore, improved service-life, assuming that suitable matrix modification occurs. This study highlights the importance to the materials scientist of understanding both the environment in which the material will operate as well as the interaction of the constituent phases. The research demonstrates that the use of these composites in the construction of everyday items such as dental fillings can be done with greater confidence in the materials.
The authors wish to acknowledge the help of the Biomaterials Unit, Chemical Engineering Department, Mechanical Engineering Department and School of Microscopy, Birmingham University, UK.