B.K. Moore

Materials Science Effect of Filler Content and Size on Properties of Composites

Two series of dental composites, along with the unfilled resin matrix, were examined to determine the effects of filler level and size on selected properties. Both series were prepared by incorporating a silanated barium borosilicate filler into a visible-light-activated polyphenylene polymethacrylate resin matrix. One series had a filler particle size of 2 μm, with filler levels of 20, 40, 45, 50, and 53% (vol). The second series contained a 15-μm filler in amounts of 20, 40, 50, 60, and 65% (vol). Tests conducted included: depth of cure as evaluated by hardness, water sorption, compressive strength, stress-strain behavior under slow compression, toothbrush abrasion, and wear by hydroxyapatite. Analysis of the data indicated that increased filler levels resulted in increased hardness, compressive strength and stiffness, and decreased water sorption. Also, there was a slight trend toward improved depth of cure. Incorporation of the 2-μm filler decreased the abrasion resistance of the resins to toothbrushing as compared with the unfilled resin, while addition of the 15-μm filler improved resistance. All filled resins exhibited a significant improvement in resistance to wear by hydroxyapatite as compared with the unfilled resin. There was a trend for increased wear with increased filler level. The particle size of the filler appeared to have a moderate influence on the properties. When compared with 15-μm filled resins of the same filler levels, the 2-μm filled series appeared to have inferior properties in terms of depth of cure, compressive strength, water sorption, and resistance to toothbrush abrasion. Properties which were less affected by particle size were hardness, stiffness, and wear resistance to hydroxyapatite.

Properties of Microfilled Composite Resins as Influenced by Filler Content

Two series of composite resins were prepared with a light-cured urethane dimethacrylate matrix to which varying amounts of two types of silanated silica particles were added. One series contained volume fractions ranging from 15.8 to 28.8% silica particles of 20 nm in diameter (Type I filler) and the other series volume fractions of from 24 to 49.4% of an agglomerated silica particle of 40 nm in diameter (Type II filler). Tests were conducted to determine the effect of filler level on: depth of cure as determined by hardness measurements; color stability in both UV light and water; water sorption with time; hardness; compressive strength; strain behavior in slow compression; and resistance to toothbrush abrasion and wear by hydroxyapatite. Analysis of the data obtained for these two micro filled series indicate that increased filler levels result in trends for increased depth of cure, color stability, hardness, compressive strength, and stiffness, while water sorption and resistance to both toothbrush abrasion and wear by hydroxyapatite were reduced. These trends were more pronounced for the Type II filler series than for the Type I filler series. However, there was a greater differential in filler levels within the Type II series than within the Type I series.

Effect of microfiller fraction and silane treatment on resin composite properties

A series of microfilled resin composites were formulated by incrementally mixing either agglomerated 20 nm or unagglomerated 50 nm silica microfillers into monomers composed of diphenyloxymethacrylate and TEGDMA. The microfiller particles were prepared with and without a gamma-methacryloxypropyl-trimethoxy silane coupling agent. Following polymerization, five material properties were tested: uniaxial tensile strength, Youngs modulus in slow compression, Knoop hardness, water sorption, and toothbrush abrasion resistance. Results from these tests indicated that microfiller content clearly was the most influential parameter affecting material property performance. Composites containing 20 nm particles demonstrated greater water sorption, higher Knoop hardness, and better resistance to toothbrush wear. Surprisingly, the application of silane to microfiller surfaces did not greatly improve composite performance for most of the material properties tested in this study. However, water sorption behavior over a 3-year period was observed to be more stable for materials possessing silane-treated particles. Future evaluation of coupling agents should include long-term water storage prior to conducting mechanical tests.

Direct restorative resins-a comparative study.

R ecently, a number of microfilled direct testotative resins have been marketed in this country. In addition, the formulations of some of the conventional composite resins have been altered. One of these composite resins, which uses strontium glass as a fillet, has been reported to be superior with respect to resistance to occlusal wear.’ The polymerization of another product is activated by visible rather than ultraviolet light. The purpose of this research was to compare some of the mechanical and physical properties of these newer commercial products with those of direct testotative resins which ate representative of systems that have been in use for an extended period of time.

The effect of relining on the accuracy and stability of maxillary complete dentures—An in vitro and in vivo study

A technique was developed for evaluating in vitro the accuracy or fit of experimental denture bases using a low-viscosity impression material for determining the space between the master die and the processed denture base. These measurements indicated that relining heat-cured maxillary denture bases with autopolymerizing acrylic resin improved their adaptation to the ridges. Interestingly, if no teeth were present in the heat-cured denture base, markedly less distortion occurred after processing. Limited clinical measurements on the stability of maxillary dentures in function showed no statistically significant change in stability after relining, but the trend was toward increased stability with the relined denture.

A comparison of the physical properties of a room temperature vulcanizing silicone modified and unmodified

RTV silicone MDX 4-4210 unmodified and modified with the addition of 5%, 10%, and 15% 360 MF 100cs was tested for the following physical properties: tensile strength, ultimate elongation (ASTM D412 die C), tear resistance (ASTM D624, die C), and Shore A Durometer hardness (ASTM D2240). The initial mechanical properties of unmodified RTV silicone were superior to those of RTV silicone modified with 5%, 10%, and 15% 360 MF except for hardness. The value for each property measured was found to decrease linearly with the amount of 360 MF added. If additional softness is desired, it can be achieved with the addition of 360 MF to RTV silicone at the expense of decreased strength, elongation, and tear resistance. Even with the addition of 15% 360 MF the values for tear resistance and elongation were in the standard range. The tensile strength of unmodified RTV silicone fell below the accepted values.

A comparison of the strength of base metal and gold solder joints

The purpose of this study was to evaluate the procedures involved in forming high- and low-fusing solder joints between two different base metal alloys and to compare the strength of these joints with the strength of those formed between specimens made from a high-gold content alloy. The conclusions of this investigation can be summarized as follows. All but 11 of the 120 specimens failed through the solder joint, which confirms that the solder joint is usually the weakest part of a fixed partial denture. The high-fusing joints formed between the base metal alloys showed higher relative bending strength values than the high-fusing joints formed between the gold alloy specimens. The low-fusing joints formed between the base metal alloys showed higher relative bending strength values than the low-fusing joints formed between the gold alloy specimens, and the low-fusing base metal joints exhibited the highest strength values in this study. The low-fusing joints formed between gold alloy specimens consistently bent before fracturing. Solder joints fail both adhesively (separation of the solder from the parent metal) and cohesively (fracture through the solder, parent metal, or a combination of both). A correctly formed joint between gold components should show cohesive fracture, whereas adhesive fractures are apparently acceptable with base metal alloys since the low-fusing base metal joints failed in this manner but still exhibited high strength values. Voids of different sizes and locations were observed in most of the solder joints, and within each group tested there was usually less strength exhibited by the joints with larger areas of porosity.

In vitro degradation of cements: A comparison of three test methods

Three in vitro tests were evaluated on the basis of their ability to predict the relative resistance of various types of luting cements to disintegration in the oral cavity. The tests were (1) weight loss, (2) reduction in size of cement films held between glass optical flats, and (3) reduction in transverse strength. Storage media evaluated were H2O and 0.1, 0.01, and 0.001 M HAc. These data were compared with previously obtained measurements on the in vivo disintegration rates of the same cements. The best correlation with in vivo disintegration was obtained by test No. 2 when the specimens were subjected to 0.01 M HAc.

Rheology of Luting Cements

A rheometer has been developed to simulate the extrusion of luting cement from beneath a full crown during seating using clinically realistic shear rates. Five luting cements were studied, and differences in rheological behavior were illustrated. These measurements demonstrate the importance of consideration of the effects of shear rate on viscosity when evaluating a cement for optimal clinical utilization.