Inclusion Of Fillers Research Articles

Cellular differentiation, bioactive and mechanical properties of experimental light-curing pulp protection materials

ObjectiveMaterials for pulp protection should have therapeutic properties in order to stimulate remineralization and pulp reparative processes. The aim of this study was to evaluate the mechanical properties, biocompatibility, cell differentiation and bioactivity of experimental light-curable resin-based materials containing bioactive micro-fillers. MethodsFour calcium-phosphosilicate micro-fillers were prepared and incorporated into a resin blend: 1) Bioglass 45S5 (BAG); 2) zinc-doped bioglass (BAG-Zn); 3) βTCP-modified calcium silicate (β-CS); 4) zinc-doped β-CS (β-CS-Zn). These experimental resins were tested for flexural strength (FS) and fracture toughness (FT) after 24h and 30-day storage in simulated body fluid (SBF). Cytotoxicity was evaluated using MTT assay, while bioactivity was evaluated using mineralization and gene expression assays (Runx-2 & ALP). ResultsThe lowest FS and FT at 24h was attained with β-CS resin, while all the other tested materials exhibited a decrease in FS after prolonged storage in SBF. β-CS-Zn maintained a stable FT after 30-day SBF aging. Incorporation of bioactive micro-fillers had no negative effect on the biocompatibility of the experimental materials tested in this study. The inclusion of zinc-doped fillers significantly increased the cellular remineralization potential and expression of the osteogenic genes Runx2 and ALP (p<0.05). SignificanceThe innovative materials tested in this study, in particular those containing β-CS-Zn and BAG-Zn may promote cell differentiation and mineralization. Thus, these materials might represent suitable therapeutic pulp protection materials for minimally invasive and atraumatic restorative treatments.

Poly (ether ether ketone) - Silicon carbide composite adhesives for elevated temperature applications of stainless steel joints

In the present work, the performance of a series of adhesives based on poly (ether ether ketone) (PEEK)/reinforced with micro-sized silicon carbide (SiC) particles for metal-metal joints was investigated. The size effect for SiC particles was studied by developing two composite adhesives with 3 wt. % of nanoparticles (NPs) (50–60 nm), and other with an equal amount of microparticles (MPs) (20 μm). The influence of particles on retention of adhesive strength at elevated temperature was studied in detail. The inclusion of MPs increased the bond strength of adhesives almost by 2 times and 15 wt. % was found to be optimum. At elevated temperatures, the adhesive strength decreased for all compositions. Interestingly the adhesive strength of composite adhesive at 300 °C was similar to the adhesive strength of virgin PEEK at ambient temperature. MPs performed better than NPs, which was correlated to agglomeration and shape of particles. SEM studies revealed that inclusion of hard fillers led to crack growth inhibition and resisting shearing during lap shear test of joints.

Enhancement of Dielectric Constant of Graphene–Epoxy Composite by Inclusion of Nanodiamond Particles

The dielectric properties of a graphene–epoxy composite have been enhanced by filling with nanodiamond particles (NDPs) as secondary filler along with graphene nanoplatelets (GNPs). The epoxy composite filled with only NDPs or GNPs to 0.1 wt.%, 0.3 wt.%, and 0.5 wt.% exhibited smaller dielectric constant compared with when filled with both. Hybrid epoxy composites were prepared with inclusion of both fillers to 0.05 + 0.05 = 0.1 wt.%, 0.15 + 0.15 = 0.3 wt.%, and 0.25 + 0.25 = 0.5 wt.%. Inclusion of NDPs in addition to GNPs also improved the dispersion of the latter in solution, which is attributable␣to kinetic energy transfer to GNPs and screening of van der Waals forces between GNPs. The enhanced dielectric constant after inclusion of NDPs is due to improved dispersion of GNPs in the epoxy matrix, which may increase the interfacial polarization.

The Effect of Li Ion Dissociation Level in Gel Polymer Electrolytes on Electrochemical Performance of Li-O2 Batteries

Li-O2 batteries have been recently considered as next generation of energy storage systems for electric vehicle application due to their high specific energy densities, which are comparable to gasoline [1]. Li-O2 batteries however suffer poor cycling and reachability due to a sever electrolyte decomposition [2]. It has been shown that the electrolyte decomposition by-products such as lithium carbonate species gradually cover the surface of cathodes and shuts down the batteries quickly [3]. Although many attempts have been made to overcome the detrimental formation of these carbonate species on the surface of cathode, their formation and electrolyte instability still remains a very big challenge in this filed [4]. Recently, it has been reported that tailoring the coordination of Li ion and solvent molecules in Li-O2 battery electrolytes is a promising method to improve the electrolyte stabilities [5,6]. In this study, we report that the inclusion of inorganic fillers in gel polymer electrolytes using different salt concentration can reduce the electrolyte decomposition by-products such as lithium carbonate species during the Li-O2 battery cycling. The improved electrochemical performance of Li-O2 batteries can be attributed to the interaction of filler with Li salt which in turn can tailor the interaction of Li ion and solvent molecules. To asses our hypothesis different electrochemical tests including in-depth electrochemical impedance spectroscopy, lithium transference number measurements along with other characterization methods such as Raman spectroscopy has been used. [1] G. Girishkumar, B. McCloskey, A.C. Luntz, S. Swanson, W. Wilcke, Lithium-air battery: Promise and challenges, J. Phys. Chem. Lett. 2010,1, 2193–2203. [2] B. D. McCloskey, C. M. Burke, J. E. Nichols and S. E. Renfrew, Chem. Commun., 2015, 51, 12701–12715 [3] S. A. Freunberger, Y. Chen, N. E. Drewett, L. J. Hardwick, F. Bardé and P. G. Bruce, Angew. Chemie - Int. Ed., 2011, 50, 8609–8613 [4] R. Younesi, G.M. Veith, P. Johansson, K. Edstrom, T. Vegge, Lithium salts for advanced lithium batteries: Li-metal, Li-O2, and Li-S, Energy Environ. Sci.,2015, 8, 1905–1922. [5] F. Li, T. Zhang, Y. Yamada, A. Yamada and H. Zhou, Adv. Energy Mater., 2013, 3, 532–538. [6] B. Liu, W. Xu, P. Yan, X. Sun, M. E. Bowden, J. Read, J. Qian, D. Mei, C. M. Wang and J. G. Zhang, Adv. Funct. Mater., 2016, 26, 605–613.

Improvement of Scratch and Wear Resistance of Polymers by Fillers Including Nanofillers.

Polymers have lower resistance to scratching and wear than metals. Liquid lubricants work well for metals but not for polymers nor for polymer-based composites (PBCs). We review approaches for improvement of tribological properties of polymers based on inclusion of fillers. The fillers can be metallic or ceramic—with obvious consequences for electrical resistivity of the composites. Distinctions between effectiveness of micro- versus nano-particles are analyzed. For example, aluminum nanoparticles as filler are more effective for property improvement than microparticles at the same overall volumetric concentration. Prevention of local agglomeration of filler particles is discussed along with a technique to verify the prevention.

Enhancing mechanical properties of polyelectrolyte complex nanofibers with graphene oxide nanofillers pretreated by polycation

The wide application of polyelectrolyte complex (PEC) nanofibers has been restrained by their unsatisfactory mechanical properties due to high porosity. In this study, we proposed a strategy for improving the mechanical properties of PEC nanofibers using graphene oxide (GO) fillers pretreated by polycation. Chitosan/gelatin (CS/GE) was adopted as the PEC model system. Through the proposed preparation procedure, GO/CS/GE nanofibers can be obtained successfully by electrospinning. Fourier transform infrared spectroscopy (FITR) and differential scanning calorimetry (DSC) results revealed the strong interactions between GO and PEC matrix, which resulted from the synergistic effect of electrostatic attraction and hydrogen bonding. The effective dispersion of GO in CS/GE matrix was verified through TEM and XRD analysis. As demonstrated by the results of tensile testing, mechanical properties of CS/GE can be effectively enhanced with the addition of GO fillers. At 1.5 wt% filler content, GO/CS/GE nanofiber membranes achieved 358% augment of tensile strength, 175% improvement of Young's modulus and 460% boost of toughness compared with neat CS/GE, which were attributed to effective filler-matrix interactions and good dispersion of fillers. Therefore, the inclusion of GO fillers into PEC through the procedure proposed in this study is a promising strategy for mechanical enhancement of PEC nanofibers.

Thermal characterization of recycled polymer for additive manufacturing applications

Abstract This work is focused on the thermal characterization of Nylon 6 based nano-composite (NC) material. Initially, melt flow index (MFI) test confirms the qualification of this material, as an alternative material for the fabrication of FDM filament. The differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) measurements characterize the material by recording their phase and mass changes as a function of temperature. The DSC results confirmed the decrease of crystallinity with the inclusion of nano fillers but also realized that these filler particles act as a thermodynamic sink and improves its stability. The TGA analysis also demonstrated the increase in thermal stability and flame retardancy level of NC material. In addition to above scanning electron microscopy analysis visualized the dispersion of filler materials in Nylon 6 matrix.

Improving Gas Barrier Performance of Polymer Nanocomposites with Carbon Nanotube Nanofillers

Conventional materials such as metals and ceramics have been replaced by polymers over the last two decades in packaging applications due to their functionality, lightweight, ease of manufacturing and low cost. However, a limiting property of polymeric materials in food packaging is their inherent permeability to gases, including oxygen, carbon dioxide and organic vapors. This has boosted interest in developing new strategies to enhance the barrier properties of polymers. The barrier properties of polymers can be significantly enhanced by inclusion of fillers, such as clay and graphene. These nanofillers make the diffusing molecules follow tortuous pathways to pass through the nanocomposite film. As a result, a decline in permeability is obtained. In this study, polyethylene (PE) filled with carbon nanotube was synthesized via a liquid mixing process. It was found that the gas barrier properties of the composites filled with carbon nanotube are improved by 40% compared with pure polymer.

Assessment of potential of nano and micro-sized boron carbide particles to enhance the abrasive wear resistance of UHMWPE

Abstract Two series of UHMWPE composites were developed using boron carbide (B4C) particles in different amounts and sizes. Micro-series contained micro B4C particles (2–5 μm) in various wt% (0, 2, 4, 6, 8 and 10) while nano-series contained nano-particles (NPs) of size 70–90 nm in 0, 2 and 3%. Abrasive wear studies against silicon carbide (SiC) paper in a single pass condition were conducted under varying loads and speeds. Inclusion of B4C particles proved beneficial to enhance the wear resistance and optimum amounts of filler for micro and nano-series were ≈ 6–8% and 3% respectively. Scratch hardness showed fairly good linearity with the wear resistance. SEM studies confirmed that inclusion of hard fillers mainly reduced the severity of micro-cutting mechanism and hence wear.

Optimization of filler type within poly(vinylidene fluoride-co-trifluoroethylene) composite separator membranes for improved lithium-ion battery performance

Abstract Porous poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) based composite membranes filled with clays (montmorillonite, MMT), zeolites (Y zeolite, NaY), ceramics (barium titanate, BaTiO 3 ) and carbonaceous (multiwalled-carbon nanotubes, MWCNT) fillers were prepared by solvent casting at room temperature. It is shown that, the thermal, mechanical and electrochemical properties of the membranes are not significantly affected by the presence of the fillers. On the other hand, the overall electrochemical behavior of the separator membranes improves with the inclusion of fillers, with respect to the pure polymer, as shown by the increase of the room temperature ionic conductivity for the composite membranes. Furthermore, cathodic half-cells based on composite membranes showed higher capacity retention and rate performance after 50 cycles than pristine polymer membranes. It is thus concluded that filler type deeply affects membrane separator performance in lithium-ion batteries, the P(VDF-TrFE) membrane with MMT filler being the one with the best performance among the evaluated fillers.

Streptococcus mutans biofilm formation and release of fluoride from experimental resin-based composites depending on surface treatment and S-PRG filler particle fraction

To evaluate fluoride release and biofilm formation on resin-based composites (RBCs) including surface pre-reacted glass ionomer (S-PRG) filler particles. Specimens were prepared from experimental RBCs including different fractions of S-PRG fillers (0/10/30/50/70% w/v). RBCs were light cured against mylar strips (MYL), and 50% of the specimens were additionally polished to a high gloss (POL). Surface roughness (SR), surface free energy (SFE) and fluoride release were determined. Streptococcus mutans biofilm formation (SMBF) was simulated for 48 h and 120 h; adherent viable biomass was assessed using an MTT-based assay. The highest SR was identified for POL specimens manufactured from the RBC with a filler fraction of 70%. For all specimens and surface treatments, polishing caused an increase in surface free energy. For both MYL and POL specimens, increasing the filler fraction coincided with an increased release of fluoride; a higher release of fluoride was identified for POL specimens with filler fractions of 50% and 70% in comparison to their MYL counterparts. Release of fluoride was lower after 120 h than after 48 h. No differences in SMBF were identified between MYL and POL specimens with identical filler fractions after 48 h of biofilm formation; with increasing filler fractions, a tendency towards decreasing SMBF was observed. After 120 h, less SMBF was identified for POL specimens with filler fractions of 30%, 50% and 70% in comparison to corresponding MYL specimens. The inclusion of S-PRG fillers and an effective surface treatment may reduce biofilm formation on RBCs.

Structure and Interactions in Polybenzimidazole Composite Membranes for High-Temperature Polymer Fuel Cells: A Full Multinuclear Solid-State NMR Study

Fabrication of composite membranes including inorganic or hybrid fillers into polybenzimidazole (PBI) membranes is a promising strategy for improving the performance of a membrane-electrode assembly (MEA) for high-temperature fuel cells. To this aim, a full understanding of the structure and interactions in such a complex system, which includes polymer, filler, and phosphoric acid, is mandatory. In this paper, we used multinuclear magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy to investigate the inclusion of inorganic and hybrid mesoporous fillers into a pyridine-based polybenzimidazole (PBI_5N) matrix and the effects of the subsequent H3PO4 doping. Composite membranes were prepared through an innovative spray deposition method, with 30 wt % of filler loading and SBA-15 or propylsulfonic-functionalized SBA-15-type silicas as filler. 13C CP MAS NMR experiments showed the structural changes induced by the acid doping on the polymer backbone. 31P(1H) 2D heteronuclear correlation experiments confirmed strong interactions between H3PO4 molecules and imidazole protons through a hydrogen-bonding network. 29Si(1H) CP MAS experiments revealed different interactions between acid and filler particles, depending on the filler type. Here, for the first time, a complete understanding of the interactions among polymer, filler, and phosphoric acid is provided. We demonstrate that a fully inorganic mesoporous filler seems to be more suitable with respect to the hybrid homologues for increasing the proton conductivity of PBI, because of its higher affinity with H3PO4. We gave evidence of a possible proton conductivity inside the mesoporous structure of SBA-15, which became hindered in the propylsulfonic-functionalized silicas due to the presence of −SO3H moieties.

Abrasive wear performance of SiC-UHMWPE nano-composites – Influence of amount and size

Ultra-high-molecular-weight-polyethylene (UHMWPE) is extensively researched tribo-material since it shows very low coefficient of friction (µ) apart from excellent wear resistance in various wear modes viz. adhesive, abrasive, erosive, reciprocating, fretting etc. Most of the times inclusion of right type of fillers/fibres shows excellent improvement in adhesive wear mode. However, it is extremely difficult to further improve the performance of a base matrix significantly in abrasive wear mode.Nano-fillers are reported as more efficient for performance enhancement as compared to their micro-counterparts. Hence two series of composites were developed by using very hard filler viz. silicon carbide (SiC) in different amounts and sizes. Micro-series contained six composites filled with micro-SiC (wt%; 0, 2, 4, 6, 8 and 10) while the nano-series contained eight composites filled with nano-SiC (wt%; 0, 1, 1.5, 2, 2.5, 3, 3.5 and 4). Both the series were characterized for physical and mechanical properties. With increase in SiC contents for both the series scratch hardness increased. Abrasive wear studies were conducted by sliding a pin of composite against the SiC abrasive paper in single-pass linear motion with fixed speed under different loads. It was observed that inclusion of SiC proved beneficial for enhancing wear resistance (approximately 15%) and optimum amount for micro-filler was around 8%. In case of nano-composites 3.5% was observed as an optimum amount to get the lowest wear rate. Nano-fillers proved to be approximately two times more effective than the micro-fillers. Increase in hardness of composite was responsible for increase in abrasive wear resistance. SEM studies indicated that inclusion of hard fillers mainly reduced the severity of micro-cutting mechanism leading to more wear resistance.

Controlling salt and aroma perception through the inclusion of air fillers

Global dietary sodium consumption significantly exceeds the WHO recommended intake levels, although strategies are available for sodium reduction, most are partial product-specific solutions. A wider range of approaches is urgently required to enable food manufacturers to reduce sodium within processed foods. In this study, the addition of air inclusions within hydrogels has been evaluated for its ability to enhance the delivery of sodium and perception of saltiness and was shown, on a volume basis, to achieve an 80% reduction in total sodium with no loss of saltiness perception; the addition of a congruent aroma volatile was shown to enhance overall flavour perception in foamed systems. Air inclusions were shown to increase both the delivery and perception of salt and aroma, in addition to increasing overall flavour perception. This work will be of interest to both academic researchers in this field and industrialists looking for new approaches to mitigate loss of taste quality with sodium reduction.

Growth of carbon nanotubes on silica microparticles and their effects on mechanical properties of polypropylene nanocomposites

• Multiscale fillers were synthesized by growing carbon nanotube on silica particles. • Appropriate crystallinity of nanotubes with high production yield was demonstrated. • Rheological behavior of the CNT–silica–polypropylene composites was investigated. • Significant improvement was achieved in mechanical property of multiscale composite. • Two micromechanical models were employed to discuss reinforcement effects. Multiscale fillers were synthesized through growing carbon nanotubes (CNT) on silica microparticles via catalytic chemical vapor deposition process. The characterization of as-synthesized CNT demonstrated appropriate crystallinity and purity of nanotubes with a production yield as high as 400% according to the catalyst-silica weight. The CNT–silica fillers were incorporated within polypropylene (PP) matrix by melt-mixing process to investigate the rheological and mechanical properties of fabricated nanocomposites. In spite of the inclusion of multiscale fillers up to 2 wt.%, the rheological behaviors of the nanocomposites were comparable to the pristine PP with moderate enhancement. An improvement by more than 35% was achieved for elastic modulus and tensile strength of nanocomposites, which was discussed by employing two micromechanical modeling approaches. The strengthening effects of CNT–silica reinforcement on impact strength of PP was revealed by impact tests and was illustrated through fractography of nanocomposites.

Composite membranes derived from immiscible NBR/SBR blends and amphiphilic montmorillonites: permeability evaluation of these membranes for benzene and toluene in their binary mixtures

The present work deals with the study of separability potential of novel composite membranes derived from acrylonitrile butadiene rubber (NBR)/styrene butadiene rubber (SBR) immiscible blends after inducing compatibilization. The compatibilization step was achieved after the inclusion of montmorillonite fillers with a wide range of amphiphilic character. The montmorillonites can be arranged in an ascending sequence according to the increase in their organophilicity: Mont-0 Mont-75 > Mont-100. The results may be simply explained considering the compatibilization process entitles different morphological states as a function of the clay organophilicity, which was additionally evidenced by recording the changes in the corresponding glass transition temperatures (Tg) of the blend components using differential scanning calorimetry (DSC). The results could also be correlated with the associated differences in the network characteristics: e.g., crosslinking density and molecular weight between two crosslinks.

Perspectives for Titanium-Derived Fillers Usage on Denture Base Composite Construction: A Review Article

Poly(methyl methacrylate) (PMMA) is an extensively used material in dentistry because of its aesthetics, processability, and reparability. However, PMMA is still far from being ideal in fulfilling the mechanical requirements of prosthesis. PMMA-based denture base polymers exhibit low fracture resistance and radiopacity behavior. Efforts to improve the mechanical and radiopacity properties of denture base materials through inclusion of silica-based fillers are ongoing. Although silane-treated siliceous fillers are commonly used, they are not sufficiently strong. They also exhibit cracks, which either cut through the glass fillers or propagate around the filler particles. This defect occurs when the dental composites are placed in aqueous oral environment because of the hydrolytic degradation of silica-based fillers and silane-coupling agents. The clinical problem of using silanes in adhesion promotion is bond degradation over time in oral environment. In addition, silanes do not bond effectively to nonsilica-based dental restorative materials. This review presents titanium-derived fillers as alternatives to siliceous fillers. Titanate-coupling agents are found to be effective couplers in treating Ti-based fillers because of their chemical compatibility and relatively high stability in aqueous environment.

Microstructural, Physical, and Fluid Dynamic Assessment of Spinel‐Based and Phosphate‐Bonded Investments for Dental Applications

This work reports the experimental evaluation of physical and gas permeation parameters of four spinel‐based investments developed with or without inclusion of sacrificial fillers. Data were compared with those of three commercial formulations. Airflow tests were conducted from 27 to 546°C, and permeability coefficients were fitted from Forchheimer's equation. Skeletal densities found for spinel‐ (ρs = 3635 ± 165 kg/m3) and phosphate‐bonded (ρs = 2686 ± 11 kg/m3) samples were in agreement with the literature. The developed investments were more porous and less permeable than commercial brands, and the differences were ascribed to the different pore morphologies and hydraulic pore sizes of ceramic matrices. The inclusion of both fibers and microbeads resulted in increases of total porosity (42.6–56.6%) and of Darcian permeability coefficient k1 (0.76 × 10−14–7.03 × 10−14 m2). Air permeation was hindered by increasing flow temperatures, and the effect was related to the influence of gas viscosity on ΔP, in accordance with Darcy's law. Casting quality with molten titanium (CP Ti) was directly proportional to the permeability level of the spinel‐based investments. However, the high reactivity of the silica‐based investment RP and the formation of α‐case during casting hindered the benefits of the highest permeability level of this commercial brand.

Li-ion battery separator membranes based on poly(vinylidene fluoride-trifluoroethylene)/carbon nanotube composites

Porous polymer membranes of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) with different types and contents of carbon nanotubes (MWCNT) were prepared by solvent casting at room temperature. The influence of MWCNT on the morphological, thermal, mechanical and electrochemical properties of the composite membranes was evaluated. It was found that the presence of MWCNT in the polymer matrix does not affect the pore distribution but increases the average pore diameter from 9 for the pure polymer up to 60 μm for the composite samples. The overall porosity increases to ~ 82% and the electrolyte uptake to 275% for the 0.5 wt.% MWCNT filler content composite membrane. Vibration bands, thermal and mechanical properties are not significantly affected by the MWCNT content and type, the degree of crystallinity decreasing with increasing filler content. The overall electrochemical behavior of the composites is improved by the presence of the MWCNT: the room temperature ionic conductivity increases from ~ 5 × 10− 7 for the pure polymer to ~ 2 × 10− 6 S/cm for the composites and becomes more stable against temperature variations. The diffusion coefficient reach a maximum value of 3.84 × 10− 5 cm2/s for membranes with 0.2 wt.% of MWCNT. The composite membranes present a stable electrochemical operation window between − 2.0 and 4.0 V, independently of MWCNT content and type. Concluding, the inclusion of MWCNT fillers up to 0.5 wt.% in a P(VDF-TrFE) matrix improves the mechanical and electrochemical properties of this porous membrane showing adequate performance for battery separator applications.

Optimized selection of metallic fillers for best combination of performance properties of friction materials: A comprehensive study

Metallic fillers are considered as very important and almost indispensible in non-asbestos organic (NAO) friction materials as they contribute to the performance properties in various ways. In spite of the awareness of the fact that their type, size, shape, amount, and combination control the performance of friction materials, hardly any systematic efforts are devoted in this direction. Authors in their earlier studies have highlighted some of the aspects in this area. The present paper has focused on very systematic and in depth investigations on these aspects by investigating the role of three different metallic fillers with variation in amount, shape, and size on the performance properties of NAO brake-pad materials. Two series of composites named P (powdery) and F (fibrous) were developed in such a way that the variation in metallic fillers was managed as; three types (copper, brass and iron) and three amounts (0, 10 and 20wt%). Composites were characterized for physical, thermal, mechanical and tribological properties (on brake inertia dynamometer) as per the standard industrial practice. It is observed that the inclusion of metallic fillers enhanced the performance properties of composites significantly. Improved thermo-physical properties especially thermal conductivity of composites played an important role in the performance properties as copper powder based composites showed best tribo-behavior followed by brass and iron powder. In some properties 10% filler loading proved most promising while for others 20% proved beneficial. From shape point of view, powdery fillers had a clear edge over fibrous. Overall, 10% loading of copper powder proved promising in most of the properties, while composite without metallic fillers proved poorest.