4 DISCUSSION: EFFECT OF FILLERS ON COMPOSITES

The addition of fillers influenced all visual and physical properties of the composites investigated. These properties could also be adjusted by varying the concentration of the filler. In filler-rich composites, the appearance and physical characteristics of the composites (color, viscosity, compressive strength, etc.) are determined by the fill material. A large amount of filler (30% resin concentration or less) will result in a composite increasingly similar to that filler. In resin-rich mixtures (above 30% concentration), the properties of the pure resin will determine the properties (including reversibility), regardless of the fill material. It may also be concluded that when choosing a thermosetting resin for high-filler-load composite fill, an adhesive with a long pot life (approximately 30 minutes or more) is preferable. When translucency is desired, it is advised to choose a resin with an RI as close as possible to that of the marble (approximately 1.65).

4.1 DENSITY

Contrary to what is expected, the density of the filler does not directly determine the density of the composite. Instead, the microstructure of the filler should be looked at when density of the composite is considered. Fillers with hollow sphere microstructure decrease the density of the resulting composite, while fillers with solid particles increase it.

4.2 WATER SENSITIVITY

Results of the absorption test showed that hollow microstructure fillers in filler-rich composites (below 30% resin concentration) seem to encourage slow drying by retaining a small amount of water for an extended period of time (72 hours) as compared to the drying rate of natural stones (6 hours). There may be too much filler in the composite to form a continuous network of resin around the filler particles, so voids in the resin may retain some water upon drying. Alternatively, water may penetrate the hollow space inside the balloon particle of the filler, which remains trapped for a longer period of time. It should be noted that filler-rich, hollow spheres-based composites will continue to dry up to 66 hours after the complete drying of a natural stone, therefore keeping the original stone moist. Although, according to the test results, the retained moisture in the hollow sphere-based composites is relatively small, for applications such as heavily decayed stone or where susceptible to fungi or algae growth, this water retention may be a factor for consideration.

A possible explanation of the disintegrated or water-damaged samples is the very short pot life of the specific Akemi resin used, which may not have permitted proper mixing. This supposition is supported by the fact that composites of comparable concentration did not disintegrate when used in the Sikadur resin. Subsequent to the completition of this study Akemi Plastics has increased the gel time of the “clear flowing” grade polyester from 7–10 to 15 minutes. This change in the product also suggests that the importance of allowing sufficient working time for thorough mixing must not be underestimated. As the disintegration phenomenon was characteristic only in the most filler-rich samples with the Akemi resin, it is conceivable that the amount of resin available to bind the large amount of particles may have been insufficient. Some of the filler particles may have been bound only by the mechanical lock of their shape within the body of the composite sample. Such flaws in these filler-rich composites may have remained unnoticed until the water immersion test when the water penetrated and forced these mechanical locks between the particles apart. The drastic results of the immersion suggest that a water immersion test may be a simple and useful qualifying test for predicting weathering performance of composites intended for exposure in an outdoor environment. The author's impression is that a capillary rise test, if performed, might not have shown disintegration of such samples so distinctly. The described water retention and disintegration of some samples indicate that problems caused by moisture might occur if composites with similar undetected flaws are used outdoors.

Results of the drying rates suggest that most composites tested will likely appear as lighter areas on moist stone surfaces.

4.3 TRANSLUCENCY AND COLOR

Anisotropic (birefringent) substances did not lend translucency to a fill. For achieving translucency of a fill similar to that of white marble, isotropic fillers with an RI difference of 0.05 or less between the filler and the embedding resin were found to be preferable. As stated earlier, composites of anisotropic fillers will remain arble does. As all concentrations made of these fillers showed similar properties in both resins, it can be concluded that composites of calcium carbonate, mica, and Cab-o-sil fillers are of excessive compressive strength as compared to natural marble, and therefore are not recommended for compensation of decayed calcerous stones. The explanation of this phenomenon is likely to be found in the solid nature of these particles. The very small size (i.e., 0.5–8μ), rounded shape, and relatively even particle size distribution of these fills are possible contributing factors.

The other group of fillers tested—Microspheres, Eccospheres, marble dust, and Globe-o-sil—particularly in concentrations between 1% and 35%, yield composites that exhibit significantly lower compressive strength (and longer performance in the plastic region) than that of natural calcareous stones and are good candidates for fills. As opposed to the other group, particles of these fillers are hollow (Microspheres, Eccospheres, and Globe-o-sil) or irregularly shaped with relatively large particle size range (crushed marble dust). These results suggest that for mechanically reversible composite fills, fillers with hollow or irregularly shaped particles with a large range of particle size, should be preferred.