Formation of gaps at the filler–resin interface induced by polymerization contraction stress

Abstract

Objectives

To investigate the movement of resin matrix with respect to the filler particles of filled composites during their photo cure without or with polymerization contraction stress (PCS).

Methods

Two types of composites were prepared. Glass beads as macroscopic fillers were placed into the center of a bis-GMA/TEGDMA resin to make single bead-embedding “composites” and a variety of fillers of different compositions, sizes, and shapes were mixed with another bis-GMA/TEGDMA resin to make lightly filled composites. They were photo cured in a cavity constructed with an acrylic or aluminum ring sitting on a polyester strip. Bonding to the ring constrained the polymerization shrinkage and thus produced a PCS. The formation of gaps between the filler and the resin was detected by optical microscopy for the glass bead–resin systems, and by light attenuation and scanning electron microscopy (SEM) for the filler–resin composites.

Results

In general, for composites with untreated fillers, the optical microscopy and SEM revealed gaps at the filler–resin interface only when they were cured under constrained shrinkage conditions. These composites attenuated more light when cured under constrained shrinkage conditions than when under non-constrained conditions. For the composites with silane-treated fillers, no gaps were observed. Some did not show any significant difference in light attenuation when cured under either constrained or non-constrained conditions.

Conclusions

The resin tends to move away from the filler particles under the influence of PCS. Strengthening the filler–resin interaction, such as by the use of silane-treated filler, may help prevent the resin departure and thus the formation of gaps.

Introduction

Polymerization contraction stress (PCS) has been studied extensively in the field of contemporary restorative dentistry, having spawned a plethora of research papers . This is largely due to two related reasons: the unique curing configuration that produces stress and the grim consequence the stress leads to. PCS is the result of the interplay between polymerization induced contraction, confined volume for the restorative, and constrained shrinkage by dental bonding. Since it always tends to pull the restorative material away from the tooth cavity wall, PCS presents a constant threat to the integrity of the restoration.

PCS is essentially a tensile stress . As a restorative material (usually a resin composite, which will be used thereafter) cures with constrained shrinkage, it is like being stretched in order to accommodate the confined volume. This strain produces a tension and thus the tensile stress. Under a free shrinking condition, such as that with no bonding to the cavity wall, the composite will not develop PCS, at least not the type we usually refer to in dentistry. Since a composite has to bond to the cavity wall to prevent microleakage and to withstand the load, PCS is perpetually present as long as the composite shrinks upon curing. In addition to imposing a pulling stress to the cavity wall, PCS simultaneously acts as an internal tensile stress on the composite as well. In contrast to that on dental bonding, however, the effect of PCS on the curing material itself has been largely ignored. It is probably because, compared with an interfacial bonding, the composite is a much stronger material. So presumably no significant damage can be expected. From the surface, that may be true. But this does not mean the composite is not affected. Based on the results of a handful of publications , the mechanical properties, especially the strength, appeared to be lower for the composites cured with PCS than those without . For example, 30 min after light cure and with its shrinkage constrained, hybrid composite Z100 showed a tensile modulus of 3.5 GPa . In comparison, its Young’s modulus was at least 6 GPa after the similar length of cure without constraint . It is likely that the PCS may have had altered the structure of the cured composite and therefore undermined its mechanical properties in the former case.

One of the common characteristics among the aforementioned researches is that they all used filler reinforced resin composites as the restorative materials . An important parameter with such materials is the filler–resin interactions . An intimate contact between the two materials is a prerequisite for optimizing the mechanical properties. We hypothesized that, due to its volume change, the curing resin will move with respect to the filler particle and this rearrangement can alter their contacting state. Moving toward the particle will generate hydrostatic pressure onto it, improving the contact. Conversely, moving away from the particle will create gaps (voids) at the interface, which weakens the filler–resin interaction and subsequently compromises the properties. Therefore, studying the alteration of their interface induced by PCS can shed some light on its influence on the mechanical properties.

This work was intended to investigate the movement of resin matrix with respect to the filler particle during photo cure of dimethacrylate based resins and their slightly filled composites. Its occurrence was inferred by the formation, or the lack, of gaps between the filler and resin. Three techniques were utilized to detect the change of interface before and after cure: (1) Gap formation between a macroscopic bead and the resin by an optical microscope, (2) Light attenuation by gap-enhanced scattering by a radiometer, and (3) Gap formation between filler particles and the resin by a scanning electron microscope (SEM).

Materials and methods

Table 1 lists the materials used in this study. The glass bead, served as a macroscopic filler particle for direct visualization, was made from a glass fiber (0.3 mm in diameter) by flaming one of its ends with a propane torch. The resin composition comprised of a typical dimethacrylate blend used for commercial dental composites. The bonding agents created a constrained shrinkage condition by bonding the composite to the ring while the separating agent freed the shrinkage by preventing the composite from adhering to the ring.

Table 1
Materials used.
Material Attribute Manufacturer
Glass bead Borosilicate Bisco, Inc., Schaumburg, IL
Resin Bis-GMA a /TEGDMA b (50:50 for beads and 60:40 for fillers) plus 0.50% CQ c and 0.50% EDMAB d Bisco
Bonding agent All Bond 2 One Step Bisco Bisco
Separating agent 316 Silicone Release Spray Dow Corning, Midland, MI

a Bis-phenol A glycidyl dimethacrylate (Esstech).

b Triethylene glycol dimethacrylat (Esstech).

c Camphorquinone (Hampford Research, Stratford, CT).

d Ethyl-4-dimethylamino benzoate (Lancaster Synthesis, Windham, NH).

Table 2 shows the seven tested fillers with varying compositions, shapes, and sizes. They were either used untreated (as received) or silane treated to strengthen their interactions with the resin. A composite slurry was made by thoroughly blending each of the fillers into the 60:40 bis-GMA/TEGDMA resin by grind-mixing with a ceramic mortar and pestle. The resin was only lightly filled (20 wt% for Spherical 1, 40 wt% for Irregular 2, and 10 wt% for the rests) to allow enough light to pass through so sufficiently accurate irradiance readings could be accomplished.

Table 2
Fillers tested.
Filler Trade name Composition Manufacturer Diameter (μm) Loading (wt%)
Spherical 1 Zeeospheres W-210 Aluminum silicate 3M St. Paul, MN 3 20
Spherical 2 TC3 Silica Suzuki Yusu Industrial Osaka, Japan 1 10
Spherical 3 SP25512 Barium aluminum borosilicate 2 10
Irregular 1 GM27884 Barium glass Schott Glass Landshut, Germany 1 10
Irregular 2 Raysorb T-4000 Strontium glass Esstech Essington, PA 10 40
Irregular 3 EG2933 Strontium glass Ferro Cleveland, OH 3 10
Nano-filler Aerosil OX-50 Silica Degussa Theodore, AL 0.04 10
All the fillers were used either as received (untreated), or silanized with γ-methacrxypropyltrimethoxy silane (silane treated).

The constraint of polymerization shrinkage was provided by a circular cavity, constructed by sitting either an acrylic or aluminum ring (6.0 mm in inner diameter, 9.5 mm in outer diameter, and 2.0 mm in height) on a clear polyester strip and covered with another strip. Because of its high compliance, the polyester strip deformed easily under PCS, contributing little constraint to the shrinkage of a curing resin or composite. Therefore, the top and bottom of the ring can be approximately treated as unbonded surfaces and the C-factor was 1.5 for both types of rings. Acrylic rings were either sandblasted and primed with bonding agent One Step to constrain shrinkage, or coated with the separating agent to allow “free” shrinkage (non-constraint). The aluminum ring was in addition primed with bonding agent All Bond 2 before One Step coating. All Bond 2 produced stronger bonding to the aluminum surface than One Step to better withstand the higher PCS using this more rigid material. Note that the non-constrained condition is relative to the constrained one in the context. Even with the use of the separating agent, there was still some constraint to shrinkage from interactions between the composite and the ring and between the composite and the polyester strips. But their contributions were much smaller than PCS and thus considered negligible.

For optical microscope observations, a specimen was prepared by pipetting the 50:50 bis-GMA/TEGDMA resin into a cavity formed by an acrylic or aluminum ring and then carefully dropping one glass bead (0.5–1.0 mm in diameter) into the resin near the center. No cover strip was applied for this set of experiments. Three types of glass beads were fabricated: smooth, smooth but with a nick (cut with a scalpel when the bead was still hot and pliable), and roughened (by sandblasting with 50 μm Al 2 O 3 ). They were used without treatments, coated with the separating agent, or treated with silane. Treatments for the last one included 9.5% hydrofluoric acid etching (Porcelain Etchant, Bisco), water rinsing, air drying, and silane coating (Bis-Silane, Bisco). The 50:50 GMA/TEGDMA ratio was thought to give better wetting to roughened glass beads than the 60:40 one. The specimen on a white mixing pad was placed on the platform of TechnoLook TW-TL10M microscope (Caltex Scientific, Irvine, CA). Optical images were recorded both before and after the resin was light cured for 20 s at 500 mW/cm 2 by an LED curing unit (Skylight by DMETE, Korea). Each set of experiment was conducted at least three times to ensure the reproducibility.

For the measurement of light attenuation, each of the lightly filled composites was transferred into a cavity formed by an aluminum ring, slightly over the edge of the ring, and then covered with another clear polyester strip, which reduced the thickness of the material to that of the ring (2.0 mm). Aluminum rings were exclusively used here because they would not interfere with the light measurements because of their opacity. The composite was irradiated with a halogen curing unit (VIP, Bisco) under 500 mW/cm 2 for 2 × 20 s from both top and bottom. After the strips were removed, the cured specimen was placed between the light guide of the VIP and a digital radiometer (Cure Rite 8000, EFOS, Mississauga, ON). Acting as a filter window, the cured composite attenuated the penetrating light by scattering as well as absorbing. The way of measuring the attenuated light was very similar to that used by Musanje and Darvell . The formation of gaps at the filler–resin interface increased the mismatch of refractive indices, making the material less transparent . A chemical cured composite with 10 wt% TiO 2 powder (P25, Degussa) was prepared for estimating light transmission around the peripheral of the cured composite: there could be gaps along the margin between the composite and the separating agent treated ring after the polymerization shrinkage. Four to five specimens were measured for each condition. The t -test was used to compare the means of measured light irradiance of those without and with polymerization shrinkage constraint.

Specimens for SEM observations were similarly cured as those above. But this time acrylic rings were used instead since the cured composite inside the aluminum ring had a strong tendency to debond from the ring during polishing. After cure and removal of the strips, the top of the specimen was ground into a flat surface with a 320 grit SiC paper (3M). The surface was then consecutively polished with a 1500 grit SiC paper (3M), 1 μm, and 0.3 μm Al 2 O 3 papers (both from Buehler, Lake Bluff, IL) in the absence of water. The polished specimens (two for each experimental condition) were air cleaned, sputter-coated with gold, and observed with SEM SM-510 (Topcon, Tokyo, Japan) under 15 kV.

Materials and methods

Table 1 lists the materials used in this study. The glass bead, served as a macroscopic filler particle for direct visualization, was made from a glass fiber (0.3 mm in diameter) by flaming one of its ends with a propane torch. The resin composition comprised of a typical dimethacrylate blend used for commercial dental composites. The bonding agents created a constrained shrinkage condition by bonding the composite to the ring while the separating agent freed the shrinkage by preventing the composite from adhering to the ring.

Table 1
Materials used.
Material Attribute Manufacturer
Glass bead Borosilicate Bisco, Inc., Schaumburg, IL
Resin Bis-GMA a /TEGDMA b (50:50 for beads and 60:40 for fillers) plus 0.50% CQ c and 0.50% EDMAB d Bisco
Bonding agent All Bond 2 One Step Bisco Bisco
Separating agent 316 Silicone Release Spray Dow Corning, Midland, MI

a Bis-phenol A glycidyl dimethacrylate (Esstech).

b Triethylene glycol dimethacrylat (Esstech).

c Camphorquinone (Hampford Research, Stratford, CT).

d Ethyl-4-dimethylamino benzoate (Lancaster Synthesis, Windham, NH).

Table 2 shows the seven tested fillers with varying compositions, shapes, and sizes. They were either used untreated (as received) or silane treated to strengthen their interactions with the resin. A composite slurry was made by thoroughly blending each of the fillers into the 60:40 bis-GMA/TEGDMA resin by grind-mixing with a ceramic mortar and pestle. The resin was only lightly filled (20 wt% for Spherical 1, 40 wt% for Irregular 2, and 10 wt% for the rests) to allow enough light to pass through so sufficiently accurate irradiance readings could be accomplished.

Table 2
Fillers tested.
Filler Trade name Composition Manufacturer Diameter (μm) Loading (wt%)
Spherical 1 Zeeospheres W-210 Aluminum silicate 3M St. Paul, MN 3 20
Spherical 2 TC3 Silica Suzuki Yusu Industrial Osaka, Japan 1 10
Spherical 3 SP25512 Barium aluminum borosilicate 2 10
Irregular 1 GM27884 Barium glass Schott Glass Landshut, Germany 1 10
Irregular 2 Raysorb T-4000 Strontium glass Esstech Essington, PA 10 40
Irregular 3 EG2933 Strontium glass Ferro Cleveland, OH 3 10
Nano-filler Aerosil OX-50 Silica Degussa Theodore, AL 0.04 10
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Nov 30, 2017 | Posted by in Dental Materials | Comments Off on Formation of gaps at the filler–resin interface induced by polymerization contraction stress
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