The clinical behavior of restorative resins varies from one brand to another.1,2 Part of this variation is associated with the filler, while another part is brought about by differences in the polymer mat­ rix. Regarding the latter, the strength properties of restorative resins were found to depend on the composition of the monomer3 and on the type and amount of functional groups involved in the polymerization.4

Apart   from   the   molecular   structure   of  the monomer, it is likely that its degree of conversion after polymerization will also affect the properties of the products. The residual double bonds in polymeric materials make them less resistant to degradation reactions.5

Clinical and laboratory studies of composite resin materials6 indicate their susceptibility to wear and loss of "anatomic form.7"10 Loss of surface fil­ ler particles followed by the abrasion of the exposed polymer matrix cause such wear. The abrasion mechanism of the exposed polymer mat­
rix could be due to mechanical wear and chemical degradation. Similarly, the discoloration of com­ posite resin restorations can be ascribed to chemi­ cal degradation and the presence of porosities.

The monomer BIS-GMA is not color stable, and tends to turn yellow with time. Unconverted methacrylate groups remaining in the polymerized material contribute to discoloration.11-13 It has also been suggested that the amount of residual monomer in the cured material is closely con­
nected to an adverse tissue reaction.

Infrared spectroscopy has been used to determine the remaining unreacted methacrylate groups or the residual monomer in the methacrylate polymers.

The five dental composite resin materials used in this investigation are listed in Table 1. The brands were available as two-paste systems consisting of inorganic filler and an organic phase comprising mainly different methacrylate monomers.

Tabie 1. Composite materials used.

Materia!               Manufacturer

Concise       3M, Saint Paul, Minnesota, USA

P10              3M, Saint Paul, Minnesota, USA

Cosmic        Detrey, Surrey, England

Biogloss        Detrey, Surrey, England

Isomolar      Vivadent, Schaan-Liechlenstein, West

                   Germany

Specimen Preparation

The composite resins were mixed according to the manufacturers' instructions and were polymerized in a casting mould against a flat glass plate and under a matrix strip to simulate clinical setting. The dimensions of the samples were 3 mm in diameter and 6 mm thick.

Each hardened composite specimen was finely ground into a powdered form using a fine abrasive paper. A weight of 2.5 mg of the powdered com­ posite mass was dissolved in chloroform and injected in a solution cell which has two NaCI win­ dows 1 mm apart.

Spectroscopic Measurements

Infrared spectra were recorded for each compo­ site component before the start of polymerization and after mixing at various intervals. The compo­ sites were kept at 37°C for 24 hours, then infrared spectra were recorded again and the amounts of remaining unreacted methacrylate groups were determined. The C=C stretching absorption bond at 1640 cm-1 was taken as representative of the unreacted monomers applying the base line method.

All spectra were recorded (Shimadzu IR-400, Minatto-Ku, Tokyo 108, Japan) in the range extending from 4000-650 cm-1 where the double beam optical null is the method of detecting the absorption bond.

Tensile Strength Test

Tensile strength was measured by the diametral tension method. Tests were made at room temper­ ature after the specimens had been stored in water at 37°C for seven days.

Loads were applied with a Universal Testing machine (Instron, Canton, Mass., USA) operated at a cross head speed of 5 mm per minute. Fifty specimens, ten of each type of composite material, were tested. The mean and standard deviations were calculated for each type.

Five recorded infrared spectra are shown in Figure 1 where varying amounts of the remaining unreacted methacrylate groups are present. The quantity of remaining methacrylate groups is deter­ mined in percent of the methacrylate groups origi­ nally present in the unpolymerized material. The absorption bond, at approximately 1640 cm-1 , is caused by C=C stretching vibrations which can be used suitably for quantitative determination of unsaturation. The remaining unreacted double bonds in all the investigated brands were evaluated by applying Beers-Lambert Law, where the amount of absorbance varied linearly with the concentra­ tion of the detected functional group.

Table 2 shows the quantitative data of the residual monomer as a mean of five independent determinations. The results demonstrate that com­ mercially available composite restorative resin materials exhibited different degrees of conversion 24 hours after the start of polymerization. Concise and P10 showed the least amount of residual monomer (unreacted carbon-carbon double bond) while Isomolar exhibited the highest quantity.

Table 2. Residual monomer measured after 24 hours.

Material        Residual Monomer% of remaining

                   double bonds

Concise                5.8

PIO                       7.5

Cosmic                 17.0

Biogloss               27

Isomolar                43.0

Table 3 shows the diametral tensile strength values of the investigated composites. It is evident that Concise exhibited the highest strength while Isomolar possessed the least.

Tabu 3. Diametral tensile strength of composite resins at 7 days.

Material               Diametral tensile strength (psi)

Concise                        6,574   ±      124.5

P10                              6,461   ±      37.3

Cosmic                         6,491   ±      87.8

Biogloss                       5,102   ±      116.3

Isomolar                       4,593   ±      104.5

The organic components of dental composite restorative materials consist primarily of several methacrylate monomers whose overall functional­ ity is greater than one such monomer. The copolymerization of such multifunctional vinyl monomers, under ambient conditions, leads to the formation of a cross-linked, three dimensional net­ work polymer having residual unsaturation in the form of unreacted monomer.14, 15 Therefore, the properties of this complex network copolymer will be determined not only by the chemical structure and composition of the monomer system but also by the degree of conversion.

The presence of unreacted monomers can have a plasticizing effect (i.e lowering the glass transition temperature of the polymer), thereby altering the physical and mechanical properties of dental mate­ rials fabricated from the monomer systems. In addi­ tion, the presence of pendant residual C=C unsaturated bonds can make the polymeric matrix more susceptible to oxidative degradation reactions.

The composite restorative materials investigated in this study showed varying degrees of conversion 24 hours after the start of polymerization. Concise and P10 had fewer remaining methacrylate groups, suggesting a more intensive and faster polymeriza­ tion.

In this study, the initial monomer concentration was found to influence the quantity of double bonds remaining in the materials after polymeriza­ tion.

The diametral tensile strength, when plotted against the quantity of remaining double bonds of the polymer, showed a tendency to correlate linearly. This finding is in agreement with Asmus-sen.16 This implies that mechanical properties of restorative resins are not only dependent upon the nature of the involved monomer molecules but also on the degree of conversion of the double bond, which usually leads to increased cross-link­
ing of the polymer.