Ahmed A. El-Hejazi, BDS, MSc, PhD
College of Dentistry, King Saud University, Saudi Arabia

The aim of this study was to investigate the effects of water sorption and solubility of two types of light cured restorative materials (hybrid and microfine composites). Six composite materials were selected - Silux Plux (SLX), Clear Fil Photopost (CRF), Durafil (DUF), Helio Progress (HLO), P10 (P10) and Ful-Fil (FUL). The experimental methods were based closely on the procedure recommended in the ISO 4049 (1988) standard for resin-bonded filling materials, taking into account recent revisions proposed by the relevant working group. The highest mean water sorption was exhibited by SLX (32.86 g/mm3). This was significantly higher than the rest of the materials. It was also shown that CRF had the lowest mean value (8.42 g/mm3). For water solubility, the results showed that the mean value for DUF (5.76 g/mm3) was significantly higher than the rest of the materials, while CRF (0.40), HLO (0.82), FUL (0.88) and P10 (1.18) were not significantly different from one other, except P10 (1.18) which was significantly higher than CRF (0.40). All the materials passed the standard specification of ISO 4049: 1988. In general, the sorption of microfine composites (eg. SLX, HLO and DUF) was significantly greater than that of hybrid composites (FUL, P10 and CRF). The solubility of microfines was also greater than that of the hybrid composites.

The oral environment plays an important role in the properties of the dental restorative materials.1  Water sorption may affect dental restorative materials such as composites, by compromising their physical and mechanical properties.1,2,3

The water absorbed by the polymer matrix could cause filler-matrix debonding or even hydrolytic degradation of the filler.3,4 Filler-matrix debonding in quartz filled as well as glass filled composites subjected to prolonged water exposure with thermocycling, have been reported.5 On the other hand, Calais and SÖderholm6 concluded in their investigation on the influence of the filler type and water exposure, that water may have a more detrimental affect on the strength of the matrix at the filler-matrix interface.

The influence of water sorption on three-body wear of composite resin has been reported by Yap, Teoh and Tan.2 They concluded that water sorption affects composites by increasing their wear. Æysaed and Ruyter7 investigated the water sorption and solubility of some posterior composites after 3 months in water at 370C.  Their results revealed that the leaching of inorganic ions into water from the fillers varied depending on filler composition and filler treatment.  Also they have shown that all fillers leach Silicon in water, with quartz composites being more stable than those based upon  Barium or Strontium glasses. The degradation in properties of composites exposed to acidic media has been reported to be more extensive in Barium and especially Zinc glass composites than quartz or microfilled materials.8 Three factors affected the amount of leaching; the extent of the polymerization reaction; the chemistry of the solvent, and the size and chemical composition of the resin.9

Six composite materials, all recommended by the manufacturers for use as filling materials, were investigated (Table 1). All materials were visible-blue-light-activated, except P10 which was chemically-activated and was also described by its manufacturer as a resin-bonded ceramic. The following procedure was based closely on that recommended in the ISO 4049 (1988) standard for resin-bonded filling materials, taking into account recent revisions proposed by the relevant working group.

Specimen preparation

A total of 30 samples were made, consisting of five specimen discs for each material.  Each specimen disc was 15 ± 1 mm in diameter and 1 ± 0.1 mm thick and was prepared using a Teflon mould.  The material was prepared in accordance with the manufacturer's instructions, by filling the mould with the material using a plastic spatula to condense, and covering it with a piece of  50 ± 30 m thick polyester transparent film which was placed over the mould and finally covered by a glass slide. 

By placing the exit window (5mm x 5mm) of the external energy source against the glass slide, that section of the specimen was irradiated for 60s. The exit window was then moved and another section, which overlapped the previous section of the specimen, was irradiated. This procedure was continued until the whole specimen had been irradiated. The specimens were removed from the mould and any flash if present, was removed.

Test procedure

The specimens were transferred to the desiccator containing silica gel, freshly dried for 5 hours (h) at 1300C. They were maintained in the desiccator at 370 ± 10C. After 24h, the specimens were removed and stored in a second desiccator which contained silica gel (freshly dried for 5h at 1300C) and stored at the lower temperature (room) of 230 ± 10C for 1h. The specimens were weighed using an analytical balance (Mettler Analytical Balance, Gallenkamp Mettler, E. Mettler, Zurich, Switzerland) to an accuracy of ± 0.1 mg. This cycle was repeated until a constant mass (m1) was obtained, i.e. until the mass loss of each specimen was not more than 0.2 mg in any 24h period. The specimens were immersed in distilled water of grade 2, (ISO 3696), and maintained at 370C for seven days.  After that time, the specimens were removed, washed with water, surface water blotted away until free from visible moisture, and waved in the air for 15, then finally weighed 1 minute after being removed from the water. This mass (m2) was recorded.  The specimens were placed in the desiccator using the same cycle as described above to obtain (m1). This cycle was repeated until constant mass (m3) was obtained.  Finally, the specimens were measured for diameter and thickness. This was done by taking three readings in the centre of the specimen to measure the thickness. The diameter was measured at four equally spaced points along the circumference. The mean values of thickness and diameter of each specimen, were used to calculate the volume (V) in cubic millimetres.

Calculations and expression of results

For water sorption, the values, Wsp, were calculated in micrograms per cubic millimetre for each of the specimens, by using the following equation: 

Wsp = (m2 - m3 )/ V

where: m2 is the mass of the conditioned specimen in micrograms, after immersion in water for seven days

m3 is the reconditioned mass of the specimen in micrograms and V is the volume of the specimen in cubic millimetres.

For solubility, the values for solubility, WSL were calculated in micrograms per cubic millimetre for each specimen using the following equation:

WSL = (m1 - m3) / V

where: m1 is the conditioned mass in micrograms

m3 is the reconditioned mass of the specimen in micrograms and V is the volume of the specimen in cubic millimetres.

The data on water sorption and solubility were subjected to Student-Newman-Keuls test at p<0.05.

The mean values, standard deviation, standard error and 95% confidence intervals for water sorption are presented in Table 2. The highest mean water sorption was exhibited by SLX (32.86 mg/mm3). This was significantly higher than the rest of the materials. It was also shown that CRF had the lowest mean value (8.42 mg/mm3) for water sorption.

Table 3 shows the mean values, standard deviation, standard error, and 95% confidence intervals (mg/mm3) for water solubility. The mean value for DUF (5.76 mg/mm3) was significantly higher than that for the rest of the materials, while CRF (0.40 mg/mm3), HLO (0.82 mg/mm3) and FUL (0.88 mg/mm3) were not significantly different from one another, and P10 (1.18 mg/mm3) which was significantly higher than CRF, HLO and FUL.

The ISO Standard (4049:1988) method for water sorption and solubility with measurements of mass at one week establishes minimum perfor-mance criteria. It appears from the results of water sorption and solubility that all the composite materials evaluated pass this standard. However, in this instance, the data can be examined further to illustrate significant differences between these materials.

In general, the sorption of microfine composites (e.g. SLX, HLO and DUF) was greater than that of hybrid composites (FUL, P10 and CRF). The solubility of microfines was also larger than that of the hybrid composites. The structural features which underlie these differences are as follows:

The following  conclusions  were drawn:

The author would like to thank Dr. Virendra  Dhuru for his advice and guidance in the preparation of this paper.

Address reprint requests to:

Dr. Ahmed A. El-Hejazi

P. O. Box 60169 Riyadh 11545

Saudi Arabia

Fax: +966-1-467 7428

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