الفهرس 
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Abstract
This study was carried out to investigate the effect of different light curing units, composite shade and increment thickness on the degree of conversion of new formulations of composite resin.A total of 180 composite specimens prepared from specially constructed split Teflon molds of 6 mm diameter and three different thicknesses (1, 1.5 and 2mm) were used for this study.The specimens were classified into two main groups of 90 specimens each according to the type of composite resin used (A); nanofilled composite resin (A1) and nanohybrid composite resin (A2). Each group was divided into three subgroups of 30 specimens each according to the composite shade used (B); light shade (B1), intermediate shade (B2) and dark shade (B3). Each subgroup was further divided into three divisions of 10 specimens each according to the increment thickness used (C); 1-mm (C1), 1.5-mm (C2) and 2-mm thickness (C3). Then each division was divided into two subdivisions of five specimens each according to the mode of curing regimen used (D); either Halogen-based light curing system (D1) or light emitting diode (D2).
The degree of conversion assessment for all groups was carried out using the Fourier Transform infrared spectrometer. For preparation of specimens, the specially constructed split Teflon molds were filled with the composite resin materials. The specimens were cured from the upper surface only and stored in dark at room temperature untill they were tested after 24 hours. Additional five uncured specimens of each composite were tested to compare the degree of conversion before and after curing.
The statistical analysis of the results showed a highly significant difference. The highest mean value of the percentage of the degree of conversion was 87.55%, which was recorded by the sub group of nanofilled composite of A1 shade and increment thickness of 1mm light cured with the LED light curing unit, while the lowest mean value of the degree of conversion percentage was 73.02% recorded by the sub group of nanohybrid composite of shade A4 and 2mm increment thickness light cured by QTH light curing unit. Results of the nanofilled composite were significantly higher than those of the nanohybrid. The results also revealed highly significant difference between the increment thicknesses used in the study, while no significant differences were found between the different shades and the LCUs used. Under the circumstances of this study, the following conclusions could be driven:
1. Degree of conversion of nanofilled composite is higher than that of nanohybrid.
2. The filler assembly influences the degree of conversion of composite resin.
3. The shade affects the degree of conversion of composite resin.
4. The degree of conversion of composite resin materials is influenced by the increment thickness.
5. The curing light source affects the degree of conversion of composite resin.
Under the circumstances of this study, the following could be recommended:
1. It is recommended to use nano filled composite resin cured with LED to get a higher degree of conversion.
2. Further investigations of the effect of color parameters including, shade, translucency and opacity are needed to determine the effect of each parameter separately on the degree of conversion of composite resin.