< First part

At PLM we observed too some ceramic fibers (fig. 4). Such fibers, which are part of the filler, show the same color of the amorphous matrix. We could see too other irregular particles of glass (fig. 5-7).

Fig. 5: An irregular fragment of glassy filler (up on the left) and a crystal (down on the right) dispersed into the matrix. Polarized light microscopy with the addition of the l-compensator, 320X.

Fig. 6-7: Both glassy and crystalline fillers are strongly represented into the Conquest Crystal composite. Polarized light microscopy with the addition of the l-compensator, 200X.

Differential Scanning Calorimeter Thermal Analysis
The thermal layout of the only lightcured samples (fig. 8) shows an initial exothermic peak, followed by an endothermic peak.

Fig. 6-7: Both glassy and crystalline fillers are strongly represented into the Conquest Crystal composite. Polarized light microscopy with the addition of the l-compensator, 200X.

Fig. 8: Thermogram of a specimen light cured for 40 s.

The endothermic peak is also present in the layout of the samples lightcured and then postcured with the BCG1 (fig. 9). While the samples lightcured and then postcured by Curing Unit didn’t show any endothermic peak (fig. 10).

Fig. 9: Thermogram of a specimen light cured for 40 s and then additionally cured for 10 min in H2O, at 170 °C and 8 bar of pressure.

Fig. 10: Thermogram of a specimen light cured for 40 s and then additionally cured for 15 min at 115 °C, in vacuum conditions.

The full layout has a different course for the three techniques; but all of them show a continuos exothermic degradation.

Discussion
By PLM analysis we can confirm that the resin matrix of the material in examination is completely amorphous regardless the curing process. For this reason the resin matrix appears black when observed under crossed nicols, or of the same color of the glassy filler under chalk filter: this confirms that the resin matrix is a monorefrigent (anisotropic) material, and then amorphous (Bloss, 1961). About the birefringent crystalline filler, it can be said that to this group belong only the minerals of the trigonal, tetragonal and hexagonal systems, called birefringent uniaxic minerals (Bloss, 1961). The ceramic fibers and the other irregular particles of glass observed are likely to be composed by barium borosilicate glass. During the PLM analysis we could not see colloidal silica, in our opinion due to the fact that the mean size (0,06 µm) is below the resolution power of our instrument. We are anyway supposing that colloidal silica is included into the filler, being a basic part of an hybrid composite (Phillips, 1991, Willems et al., 1992).

By DSC analysis is clearly demonstrating the absence of any crystalline status, created by the heating process; If the material would have been crystalline it would have presented, during the fusion induced by the heating, a deep endothermic peak without subsequent degradation, as in fact, was the case in all the examined situations.

For what it concerns the initial exothermic peak of the only lightcured samples (fig. 8), it could be due to an incomplete thermal balance between samples and reference; but in our opinion this peak has to be considered due to an incomplete curing of the material only by light. According to other authors (Clarke, 1988), the heating due to thermal analysis complete the partial curing in the only lightcured samples. Our statement is confirmed by the fact that this peak is practically absent in the layouts of those samples which were cured with the other two techniques (fig. 9, 10) where the material has to be considered as completely cured. In the layouts of samples which were only lightcured, after the exothermic peak there is an endothermic peak due to the loss of H2O (humidity of the sample). This peak is also present in the thermal layouts of the samples that were lightcured and then additionally cured with the BCG1 (fig. 9); this phenomenon is related to the curing conditions in watery environment.

In our opinion, in the samples postcured with the Curing Unit (fig. 10) the endothermic peak cannot be observed because the postcuring process takes place in dry environment and this has contributed to dehydrate them completely.

The differences in the final part of the thermal layouts, for the three examined techniques, can be probably debited to the difference in weight of the samples. Many authors suggest that the postcuring process can improve the homogeneity of the polymeric networks (Bausch et al., 1981; Roulet and Noack, 1991; Ruyter, 1992). This can offer a further explanation for the different course of the layouts, at the end, were an increase in the Heat Flow (HF) appears for samples postcured by BCG1 and Curing Unit (fig. 9, 10). With this last device, in particular, the highest values of HF were reached proving that the temperature of exercise and the condition of vacuum are ideal for an optimal polymerization of such OCDMA based material.

Conclusions
Our analysis demonstrates the product investigated has no crystalline properties. Due to the fact that OCDMA has a crystalline polymer structure, we can interpreter the regular alignment of OCDMA polymer is strongly disturbed by the amorphous polymer given by the other resinous components. From our findings we can conclude this material could be advantageously used for semi-indirect or indirect composite inlays, while we cannot confirm the indication for direct restorations, because of the incomplete curing by light only. The DSC analysis suggest an optimal homogenization and rearrangement of the polymeric network after tempering for 10 min at 115 °C in vacuum conditions, as for Conquest C/ B.

At PLM, we could see a crystalline filler together with the simply amorphous barium borosilicate glass: this account for a ternary filled product instead of binary filled in spite of the manufacturer’s assessments.

Bibliografy

1. Barrall EM, Johnson JF (1966). Instrumentation, techniques and applications of differential thermal analysis. In: Slade PE, Jenkins LT. Techniques and methods of polymer evaluation. Vol 1. New York: Marcel Dekker, Inc., 2-42.
2. Bausch JR, De Lange C, Davidson CL (1981). The influence of temperature on some physical properties of dental composites. J Oral Rehabil 8: 309-317.
3. Bloss FD (1961). An introduction to the methods of optical crystallography. San Francisco: Holt Rinehart and Winston, 280-310.
4. Clarke RL (1988). Dynamic mechanical thermal analysis of dental polymers bisphenol A related resins. Biomat 10: 549-557.
5. Craig RJ (1981). Chemistry composition and properties of composite resins. Dent Clin N Amer 25: 219-39.
6. Goracci G, Andreasi Bassi M, Cito C (1998). Intarsi in composito: una applicazione del fototropismo analisi in vitro. Dental Cadmos 16: 1-27.
7. Hisiao J, Vijayaraghavan TV (1992). Flexural behavior of VLC hybrid composites as function of test temperature. J Dent Res 71: 136 Abstr. No. 243
8. Peutzfeldt A (1997). Resin composites in dentistry: the monomer systems. Eur J Oral Sci 105: 97-116.
9. Phillips RW (1991). Science of dental materials. 9th ed. Philadelphia: Saunders, 169-170, 221-229.
10. Tashiro K, Tadokoro H (1989). In: Mark HF, Bikales NM, Overberger CG, Menges G. Encyclopedia of polymer science and engineering. Vol. Suppl., New York: John Wiley & Sons, Inc., 187-230.
11. Roulet JF, Noack MJ (1991). Criteria for substituting amalgam with composite resins. Int Dent J 41: 195-205.
12. Ruyter IE (1992). Types of resin-based inlay materials and their properties. Int Dent J 42:139-144.
13. Waknine S (1991). Conquest DFC: a novel universal dental composite restorative system. J Esthet Dent Update 2: 70-79.
14. Waknine S, Jia W, Prasad A, Shulmam A (1991). Direct/indirect commercial composites characterization of strength, shrinkage and wear. J Dent Res 70: 481 Abstr. No. 1722.
15. Waknine S, Goldberg AJ, Muller HJ, LeGeros J, Prasad A, Shulmam A (1992). Fracture toughness of a new semi-crystalline dental resin. J Dent Res 71: 313 Abstr. No.1660.
16. Willems G, Lambrechts P, Braem M, Celis JP, Vanherle G, (1992). Classification of dental composites: according to their morphological and mechanical characteristics. Dent Mat 8: 310-319.