P Pallav, AJ de Gee, CL Davidson, RL Erickson, and EA Glasspoole

A series of light-curing composite resins, with a Bis-GMA / TEGDMA resin matrix, was tested in the Acta Wear Machine for occlusal wear. 

On the same materials surface roughness, diametric tensile strength, and hardness were tested.

The materials contained mixtures of ground quartz (the macro filler,  about 3 µm) and colloidal silica (micro filler, about 40 nm). The total amount of filler, 68.5% by volume, was the same for all materials.

The study was conducted to determine the influence of the ratio of micro to macro filler in the mixtures. When the macro filler was gradually replaced by 3, 5, 7.5, 10, 15, and 20 vol.% of micro filler, wear resistance increased substantially. Any influence on surface roughness or tensile strength and hardness could not be demonstrated.

The reason for admixing small amounts of micro filler in a composite is to prevent settling of the macro filler and to adjust handling properties. 

Materials and methods

The series of experimental light-curing composite resins was manufactured especially for this study. 
The filler consisted of a mixture of ground quartz with an average particle size of 3 µm and colloidal silica (40 nm in diameter), both silane treated. 
The resin matrix was a 1 : 1 mixture of Bis-GMA and TEGDMA. 
The total volume of filler was held constant at 68.5 % as only the ratio of ground filler to colloidal silica was changed. 

Of these materials the following properties were tested.

• Wear in the Acta Wear Machine
• Surface roughness (Ra), 
• Diametric tensile strength (MPa), and 
• Brinell hardness number (BHN)

The results are in the figure above, except the roughness values, which are in the table. 

Discussion

Besides the necessity of admixing small amounts of colloidal silica, in the order of 3 - 4 % by volume, to prevent settling of the ground filler and to adjust handling properties, there also appears to be a favorable effect on the wear properties. 

The exact reason for the improved wear resistance with increasing micro filler isn't clear. Probably the increased Young's modulus of the matrix offers improved support for the macro filler particles due to a more even distribution of local stresses around these. The gain was largest for the first 15 % by volume, after which the wear remained constant. 

An improved resistance to brushing wear, when the micro filler level is raised, was found earlier in brush wear experiments on micro filled composites by De Gee et al. (1984 and 1985) and St. Germain et al. (1985). However these authors found an optimum at a micro-filler level of approximately 15 to 20 vol.% (15 to 20 vol.% in the resin matrix translates to approximately 5 to 7.5 vol.% of micro-filler in the tested materials, cf. the table). At higher concentrations the wear resistance decreased again. This is explained (De Gee et al., 1984) by increasing clustering of micro-filler particles, resulting in insufficient particle-matrix coupling within the clusters and therefore insufficient cohesion. Although the improvement of wear resistance definitely came to an end between A6 and A7, an optimum was not found in the present study, probably because clusters are much more easily disrupted by the macro-filler particles during the manufacturing procedure of the pastes. Clustering and wetting problems apparently only occur beyond 20 vol.% of micro-filler (approximately 40 vol.% of micro filler in the resin phase, cf. the Table 1) as the viscosity of the paste becomes too high. For compositions such as A5 and A6, good paste handling properties were still present. 

In contrast to the positive effect of micro filler on the wear characteristics, its presence in the formulations did not affect the surface roughness. Apparently the macro filler is mainly responsible for the surface roughness.
Whereas mechanical properties do depend on total filler load (Braem et al., 1987b), no influence of varying the macro-to-micro filler ratio was found on the diametrical tensile strength or hardness (apart from a slight increase with the first 3 vol.% of micro filler). From these findings it can be concluded that mechanical properties such as (Brinell-) hardness or diametrical tensile strength do not provide useful information to the development and evaluation of dental composite resins with respect to wear. 

Based on the article:
P Pallav, AJ de Gee, CL Davidson, RL Erickson*, and EA Glasspoole*: The Influence of Admixing Microfiller to Small-Particle Composite Resin on Wear, Tensile Strength, Hardness and Surface Roughness. J Dent Res 68:489-490. *= 3M Company, St. Paul, MN, USA.