EFFECTS OF FILLER, INITIATOR AND CAVITY DESIGN FACTOR ON POLYMERIZATION STRESS DEVELOPED IN DENTAL COMPOSITES

Introduction

Since the introduction of acrylics into dentistry in the 1960’s (1), considerable developments in filler technology, resin and initiation systems, as well as improvements in the adhesion of dental composites to tooth structure have significantly expanded their clinical utility. However, in spite of improvements in bonding properties, micro-leakage and gap formation primarily at the dentin/composite interphase remain major weaknesses of these materials (2). During photo-polymerization of composites network (gel point) quickly forms within the resin phase. The composite’s elastic limit reaches a level that does not allow enough relaxation to occur to compensate for the reduction in volume and the rapid buildup of stress. Any additional polymerization shrinkage beyond the gel point adds to this internal stress that develops in the polymer matrix and at the interphase with the filler phase. For composites bonded to teeth, polymerization shrinkage is less free and polymerization stress development (PSD) becomes more complex due to the additional external stresses, usually unevenly distributed along the cavity walls and the bonded composite surface (3, 4).

A number of material as well as processing factors can contribute to PSD in composites. Filler type and content, resin type and composition and mode of polymerization determine the amount of volumetric shrinkage, elastic modulus and PSD of the material (5). The polymerization process is affected by type and concentration of initiators, e.g. chemical vs. photochemical, which determine reaction kinetics and degree of vinyl conversion (DVC; 6). PSD values also vary according to the ratio of the bonded to the un-bonded (free) surface area of the composite in a cavity, i.e., the configuration or C-factor (7). The hypothesis that a large free surface area (lower C-factor value) would lead to lower PSD values was confirmed by Feizler et al. (8). The reduction in PSD is attributed to the fact that such large un-bonded areas of composite allow greater plastic deformation to occur during polymerization before the gel point is reached.

The objective of this study was to assess the effect of type of filler, initiator system and C-factor on PSD in resin composites by tensometry.

Experimental

Formulation of the resin and composites

The experimental resin was prepared from the commercially available monomers: 2,2-bis[p-(2′hydroxy-3′-methacryloxypropoxy)phenylene]propane (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) in a 1:1 mass ratio (designated BT resin). Two series of composites were formulated by inclusion into the BT resin 1) a mass fraction of 73.3 % of silanized glass filler (average particle size 0.7 μm; BT/glass composite), or 2) a mass fraction of 40 % zirconia-hybridized amorphous calcium phosphate (ACP) filler (average particle size 5.7 μm; BT/ACP composite). The latter is an experimental composite with demonstrated remineralizing potential (9). A commercial composite was used in the C-factor study as a control.

Polymerization stress development (PSD) measurements

PSD was quantified by utilizing a computer-interfaced, cantilever beam tensometer developed at Paffenbarger Research Center, ADAF, at NIST, Gaithersburg, MD ((10); Fig. 1). The deflection of the cantilever beam was measured with a linear variable differential transformer. The tensile force was calculated from a beam length (12.5 cm) and a calibration constant (3.9 N/μm). PSD was obtained by dividing the measured tensile force by the cross sectional area of the sample (diameter = 6 mm). A minimum of three measurements were made for each experimental group.

Fig. 1.

A schematic of tensometer used to measure the PSD of composites (10).

Effect of initiator system on PSD

Initiator effects on PSD were evaluated in BT/glass composites activated for polymerization as indicated in Table 1. The experiments were performed at a constant C-factor = 1.33.

Table 1.

Components of the photo and chemical initiator systems utilized to formulate BT/glass composites

Initiator type	Chemical name	Acronym	Concentration (mass fraction %)
Photo	Camphorquinone	CQ	0.2; 0.4
	Ethyl-4-dimethyl-aminobenzoate	4EDMAB	0.8; 1.6
	Phenyl-bis(2,4,6-trimethyl benzoyl) phosphine oxide	819	0.5
Chemical	Benzoyl peroxide	BPO	2.0
	N,N-dihydroxyethyl-p-toluidine	DHPT	1.0

The photo-activated composites (CQ+4EDMAB and “819” in BT/glass series, BT/ACP composites and commercial composite (THP; Caulk Dentsply, York, PA, USA)) were irradiated with a visible light (curing unit, Dentsply, York, PA, USA) for 60 s to initiate polymerization, and the PSD was then measured at 60 min. For the chemically initiated BT/glass specimens, the BPO and DHPT pastes were mixed in 1:1 mass ratio for 1 min before being inserted into the sample chamber of the tensometer, and their PSD was measured at 60 min and 120 min.

Effect of test geometry (configuration or C-factor) on PSD

To assess the effect of the test geometry, generally characterized as C-factors, the heights (h) of the photo-activated (CQ and 4EDMAB at mass fraction 0.2 % and 0.8 %, respectively) BT/ACP composite as well as that of the control TPH composite specimens were systematically varied between 0.5 mm and 3.75 mm to give C-factor ranging from 6.0 to 0.8.

C-factor was calculated as the ratio of bonded composite area (2πr²)/un-bonded area (2πrh), where r is the radius of the quartz rod cylinder and h is the height of composite cylindrical specimens. The measured PSD values for specimens with variable C-factors were normalized to specimens with C-factor of 1.33 (h=2.25 mm) as controls to give calculated PSD values.

Degree of vinyl conversion (DVC)

The DVC attained in the TPH control and in the experimental BT/ACP and BT/glass composites (activated with CQ and 4EDMAB at 0.2 % and 0.8 % mass fraction and containing mass fraction 40 % filler) was measured by near-infrared (NIR) spectroscopy (11). NIR scans (Nicolet Magna 550, Nicolet Inc., Madison, WI, USA) were taken before photo cure and 1 min and 2 hr post-cure. Composites with a thickness of 3.0 mm (C-factor = 1.00) were compared. DVC was calculated as the % change in the integrated peak area of the 6165 cm^-1 methacrylate vinyl absorption band between the cured and uncured composite normalized to the specimen thickness.

Statistical Analysis

Experimental data were analysed by ANOVA (α=0.05). Significant differences between specific groups were determined by all pair-wise multiple comparisons (Tukey test). One standard deviation (SD) is taken as a measure of the standard uncertainty of the measurement.

Results and Discussion

The first study explored the effects of the polymerization initiation mode on the PSD in the BT/glass composites (Table 2).

Table 2.

PSD measured in BT/glass composites. Results indicate mean value ± standard deviation (SD) of three replicate runs in each group. Post-curing time: 60 min or 120 min; C-factor = 1.33

Initiator system (mass fraction %)	PSDmeasured (MPa)
	60 min	120 min
CQ + 4EDMAB (0.2 +0.8)	4.46 ± 0.17	4.46 ± 0.17
CQ + 4EDMAB (0.4 +1.6)	4.78 ± 0.17	4.78 ± 0.17
819 (0.5)	3.85 ± 0.16	3.85 ± 0.16
-----------------------------------------	---------------------------------------------
BPO + DHPT (2.0 + 1.0)	3.47 ± 0.31	3.68 ± 0.28

The above results indicate that initiator system can have significant influence on the PSD of composites. Presumably tensometer studies can be used to help identify the type of initiator systems and the concentration of initiators that may best modulate the PSD of dental composites. As expected, the chemical initiator compared to the photochemical initiators slows the kinetics of polymerization yielding a significantly lower PSD even after 2 hr.

The second study explored the influence of test geometry (C-factor) on the PSD in TPH composites and BT/ACP composites. The results show similar trends and are summarized in Tables 3a and 3b.

Table 3a.

Measured and calculated PSD in TPH composite specimens as a function of C-factor. Shown are the mean value ± SD of three measurements for each group (PSD_measured) and the average PSD_calculated

C-factor	Composite height (mm)	PSDmeasured (MPa)	PSDcalculated (MPa)
0.80	3.75	2.78 ± 0.07	1.67
0.86	3.50	2.91 ± 0.05	1.87
1.00	3.00	2.70 ± 0.07	2.03
1.33	2.25	3.16 ± 0.19	3.16
2.50	1.20	3.37 ± 0.08	6.32
6.00	0.50	2.82 ± 0.17	12.69

Table 3b.

Measured and calculated PSD in BT/ACP composite specimens as a function of C-factor. Shown are the mean value ± SD of three measurement for each group (PSD_measured) and the average PSD_calculated

C-factor	Composite height (mm)	PSDmeasured (MPa)	PSDcalculated (MPa)
0.86	3.50	5.80 ± 0.55	4.39
1.00	3.00	6.21 ± 0.27	4.66
1.33	2.25	6.55 ± 0.19	6.55
2.50	1.20	6.79 ± 0.34	12.73
3.00	1.00	6.96 ± 0.06	14.73
6.00	0.50	6.83 ± 0.68	26.09

In both the TPH and BT/ACP composite series, PSD_calculated increased with the increasing C-factor, confirming the hypothesis that a large free area (lower C-factor value) leads to lower PSD values (8). The higher PSD_measured and PSD_calculated values for the experimental BT/ACP composite compared to the commercial TPH composite probably reflect differences in the type and mass of the resin and filler phases in the two types of composites.

The DVC values attained in BT/ACP composites are summarized in Table 4.

Table 4.

DVC (mean value ± SD) attained in various composites. Number of runs ≥ 4/group. Specimen thickness = 3 mm

Composite type	DVC (%)
	1 min post cure	2 hr post cure
BT/ACP	67.47 ± 2.23	69.46 ± 2.77
BT/glass	64.57 ± 2.31	68.12 ± 5.12
TPH	54.99 ± 2.69	57.48 ± 1.98

Higher DVCs attained in BT/ACP composite 2 hr after cure compared to TPH composite indicate that DVC is another important factor that influences PSD in dental composites.

Conclusions

Tensometry can be utilized to identify optimal initiator systems that yield favorable PSD values in dental composites. Also, cavity configuration or C-factor needs to be considered in minimizing PSD values to improve the quality of the interphase between composite and tooth structure. Overall, tensometry has the potential to be a useful tool in optimizing material and processing factors for developing polymeric materials with favorable PSD values.