1

Introduction

The use of dimethacrylate resin based composites for restoration of posterior teeth has rapidly increased due to patient interest in esthetic restorations and the disadvantages associated with dental amalgam . In posterior teeth, composite resin restorations have a median survival of 5 years compared to 15 years for dental amalgams . Failures of posterior composite restorations have been attributed to marginal or bulk fractures , and/or secondary caries . Improvements in fracture toughness would be expected to enhance the clinical performance and survival rate of dental composites .

Dental composite matrices are composed of thermoset resins, principally, bisphenol A-glycidyl methacrylate (Bis-GMA) and other dimethcrylate diluents. The polymerized resins are highly crosslinked, brittle and have poor impact strength . Consequently, several methods of improving mechanical properties have been reported. These methods include, changes in resin composition , filler particles , and heat treatment . In industry, toughening of thermoset polymers has been achieved by incorporating low molecular weight liquid rubbers (LRs) . Several functionally terminated LRs have been used as toughening agents. On curing, the LRs phase separate to form rubbery microdomains in the crosslinked polymeric matrix of the thermoset resin . The rubber microdomains increase the toughness of the crosslinked thermoset. Similar attempts have been made in dentistry to toughen composite resins using a variety of butadiene based rubbers . However, the poor solubility of butadiene monomers in Bis-GMA based resins has limited their effectiveness as tougheners. According to a recent patent by La Fleur , the poor miscibility of LR’s in thermoset resins is attributed to the presence of end-functionalized polymer chains on the LR’s, which tend to crosslink and increase molecular weight and viscosity, while reducing miscibility in the thermoset resin. Additionally, the terminal functional groups cause the LR to react prematurely with the liquid thermoset resins prior to cure. The miscibility of LR’s in thermoset resins can be improved by the addition of at least one non-functional aromatic end-group to the polymer chains of such LR compositions . A novel acrylate copolymer LR with functional end groups blocked by reaction with phenol was investigated as an additive for Bis-GMA resins. The aim of this investigation was to determine the effect of addition of low concentrations of the novel experimental LR on the fracture toughness of a 50/50 wt% Bis-GMA and TEGDMA polymer. The effect of the LR on crosslink density and Barcol hardness of the polymerized mixture were also determined.

2

Materials and methods

The LR used is 2-methyl-2-propenoic acid telomer with butyl 2-propenoate, dimethylbenzene, (Rohm & Haas, Philadelphia, PA). The liquid rubber was prepared by combining n-butyl acrylate (BA), methyl methacrylate (MMA) and glycidyl methacrylate (GMA) in a weight ratio of 90:05:05. Polymerization was carried out in a xylene solvent with di-tertiary butyl peroxide as an initiator. Characterization by NMR spectroscopy showed that the product is comprised of approximately 0.72 mole of terminal units, and 30.75 mole of internal units derived from BA, 1.30 mole of internal units derived from GMA, 1.65 mole derived from methyl methacrylate and 1.30 mole of terminal units derived from xylene. Benzyl groups (derived from xylene) were incorporated into the polymer Fig. 1 . No initiator fragments were detected in the polymer chain.

Chemical structure and composition of experimental liquid rubber. BA, n-butyl acrylate; MMA, methyl methacrylate;

GMA, glycidyl methacrylate.

The LR was added to a solution of Bis-GMA (50 wt%) and TEGDMA (50 wt%) (Esstech, Essington, PA) containing 1 wt% N,N-dimethylamino ethyl methacrylate and 0.5 wt% camphoroquinone (Acros Organics, Geel, Belgium).

Bis-GMA/TEGDMA compositions containing 0–10% LR at 2% increments were fabricated selected for fracture toughness measurements using notched bars according to ASTM 399. The mixtures were polymerized by exposure to a visible light curing unit Optilux 500 (SDS Kerr; Danbury, CT) with a light intensity of 500 mW for 60 s. Ten single edge-notched bar samples, 25 mm × 5 mm × 2.5 mm of each composition were fabricated in a metal mold and stored for one week at 37 °C and 100% relative humidity. Fracture toughness was determined in 3-point loading using a universal testing machine (Instron Corp., Canton, MA) at a crosshead speed of 0.5 mm/min. Fracture surfaces were sputter coated with gold using a Denton Desk II Sputter Tabletop DC magnetron sputter coater (SPEC, Santa Clara, CA) and an Analytical Scanning Electron Microscope (JEOL (6400 SEM) Ltd., Tokyo, Japan) was used to qualitatively observe the distribution of rubber in the specimens.

Specimens for investigating crosslink density were fabricated by adding the experimental LR at concentrations used for fracture toughness measurements. Disk shaped specimens 6 mm diameter by 4 mm thickness of Bis-GMA/TEGDMA/LR composites were fabricated in a metal mold. Specimens were stored for 7 days at 37 °C. After storage, 10 specimens for determination of crosslink density were weighed and then immersed in 75% ethanol solution. Specimens were blotted dry and weighed after 24 h of immersion. The percentage change in weight before and after immersion in ethanol was compared between LR containing samples and a BIS-GMA/TEGDMA control.

Ten new specimens of compositions investigated for fracture toughness were fabricated for hardness measurements. Specimens were the same dimensions as those used for determination of crosslink density. Barcol hardness was determined using a Barcol Hardness Indentor Model GYZJ 935 (Barber Colman Co., Loves Park, IL) following ASTM D2583. Three indentations were made at the top and underside of each specimen after storage for 7 days. Data obtained for fracture toughness, degree of crosslinking and Barcol hardness were separately evaluated using one-way ANOVA with LR composition as independent variable. The level of significance was set at p < 0.05.

2

Materials and methods

The LR used is 2-methyl-2-propenoic acid telomer with butyl 2-propenoate, dimethylbenzene, (Rohm & Haas, Philadelphia, PA). The liquid rubber was prepared by combining n-butyl acrylate (BA), methyl methacrylate (MMA) and glycidyl methacrylate (GMA) in a weight ratio of 90:05:05. Polymerization was carried out in a xylene solvent with di-tertiary butyl peroxide as an initiator. Characterization by NMR spectroscopy showed that the product is comprised of approximately 0.72 mole of terminal units, and 30.75 mole of internal units derived from BA, 1.30 mole of internal units derived from GMA, 1.65 mole derived from methyl methacrylate and 1.30 mole of terminal units derived from xylene. Benzyl groups (derived from xylene) were incorporated into the polymer Fig. 1 . No initiator fragments were detected in the polymer chain.

Chemical structure and composition of experimental liquid rubber. BA, n-butyl acrylate; MMA, methyl methacrylate;

GMA, glycidyl methacrylate.

The LR was added to a solution of Bis-GMA (50 wt%) and TEGDMA (50 wt%) (Esstech, Essington, PA) containing 1 wt% N,N-dimethylamino ethyl methacrylate and 0.5 wt% camphoroquinone (Acros Organics, Geel, Belgium).

Bis-GMA/TEGDMA compositions containing 0–10% LR at 2% increments were fabricated selected for fracture toughness measurements using notched bars according to ASTM 399. The mixtures were polymerized by exposure to a visible light curing unit Optilux 500 (SDS Kerr; Danbury, CT) with a light intensity of 500 mW for 60 s. Ten single edge-notched bar samples, 25 mm × 5 mm × 2.5 mm of each composition were fabricated in a metal mold and stored for one week at 37 °C and 100% relative humidity. Fracture toughness was determined in 3-point loading using a universal testing machine (Instron Corp., Canton, MA) at a crosshead speed of 0.5 mm/min. Fracture surfaces were sputter coated with gold using a Denton Desk II Sputter Tabletop DC magnetron sputter coater (SPEC, Santa Clara, CA) and an Analytical Scanning Electron Microscope (JEOL (6400 SEM) Ltd., Tokyo, Japan) was used to qualitatively observe the distribution of rubber in the specimens.

Specimens for investigating crosslink density were fabricated by adding the experimental LR at concentrations used for fracture toughness measurements. Disk shaped specimens 6 mm diameter by 4 mm thickness of Bis-GMA/TEGDMA/LR composites were fabricated in a metal mold. Specimens were stored for 7 days at 37 °C. After storage, 10 specimens for determination of crosslink density were weighed and then immersed in 75% ethanol solution. Specimens were blotted dry and weighed after 24 h of immersion. The percentage change in weight before and after immersion in ethanol was compared between LR containing samples and a BIS-GMA/TEGDMA control.

Ten new specimens of compositions investigated for fracture toughness were fabricated for hardness measurements. Specimens were the same dimensions as those used for determination of crosslink density. Barcol hardness was determined using a Barcol Hardness Indentor Model GYZJ 935 (Barber Colman Co., Loves Park, IL) following ASTM D2583. Three indentations were made at the top and underside of each specimen after storage for 7 days. Data obtained for fracture toughness, degree of crosslinking and Barcol hardness were separately evaluated using one-way ANOVA with LR composition as independent variable. The level of significance was set at p < 0.05.