The mechanical stability of nano-hybrid composites with new methacrylate monomers for matrix compositions

Abstract

Objectives

Dimer acid based metacrylates and TCD-urethane are promoted as new monomers of nano-hybrid resin based composites as alternatives for the conventional BisGMA. Investigations of this study focused on the mechanical and the storage behavior of nano-hybrid resin based composites (RBCs) composed of these new types of monomers in comparison to RBCs using BisGMA.

Methods

Flexural strength and modulus were determined in a three-point-bending test. Additionally, the modulus of elasticity was measured on microscopic scale ( E micro ) using an automatic microhardness indenter. Tests were performed on samples after 24 h storage in distilled water, as well as after thermocycling and storing the materials for four weeks in either distilled water, artificial saliva or ethanol.

Results

The six measured materials showed a pronounced decrease of flexural strength, flexural modulus and E micro after four weeks storage in alcohol. Results after four weeks storage in water and saliva could not be proven to be significantly different. The most sensitive factor of influence on all test parameters was the material.

Significance

Nano-hybrid composites with new or conventional monomers performed similar in regard to the mechanical properties and the behavior of the materials after aging.

Introduction

The current trend toward minimizing filler size in order to achieve great optical qualities, and toward maximizing filler loading is an attempt to satisfy all of the requirements for dental composites . Advantages named within the context of high filler loading of RBCs are improved mechanical properties , high wear resistance , and reduced polymerization shrinkage . Nano-hybrid RBCs contain a range of different filler sizes, also large filler particles besides the eponymous nano scale sized fillers. The varying particle sizes provide for a homogenous filler distribution within the matrix, since the small nano fillers are able to occupy the spaces between the larger particles perfectly and therefore help to generate RBCs with filler loadings that are comparable with the conventional hybrid composites. Nano-hybrid RBCs are claimed to combine both the positive characteristics of macro-filled composites (such as excellent physical and mechanical properties) and of micro-filled ones (e.g. excellent finishing and polishing qualities) and thus can be recommended as universal filling materials for anterior and posterior restorations . The matrix of most of these new types of composites still consists of the conventional BisGMA monomer developed by Bowen, yet new types of monomers have recently been introduced in the matrices of nano-hybrid composites, like the dimer acid based dimethacrylate monomer (N’Durance, Septodont), and a special urethane monomer, namely TCD-urethane (Venus Diamond, Heraeus Kulzer). The core structure of the dimer acid based monomer is composed of both linear and cyclic aliphatic structures . “Dimer acid” means any of the class of cycloaliphatic carboxylic acids that are high-molecular-weight dibasic acids which are liquid (viscous), and which can be polymerized directly with alcohols and polyols to form polyesters . The manufacturer promises both low volumetric shrinkage, and a high conversion rate. Two characteristics that do not seem compatible, but desirable in order to reduce stress on the tooth and in case of the conversion rate limit the elution of any residual monomer and thus enhance the biocompatibility of the material. This controversy is explained by the bulky nature of the core structure of the dimer acid based dimethacrylates. Because of their high molecular weight and a low initial double bond concentration, dimer acid monomers still have significantly lower polymerization shrinkage, despite of the high conversion rate achieved .

Only little information is available about the TCD-urethane monomer. According to information provided by the manufacturer, a low shrinkage crosslinker monomer with a special aliphatic structure has been synthesized. The new methacrylic acid derivatives, containing urethane groups of tricyclo-decanes are prepared by reaction of hydroxyalkyl (meth)acrylic acid esters with diisocyanates and subsequent reaction with polyols . Similar to bisphenol-A, the structure of the TCD-urethane backbone was proven to be rigid . In combination with the high reactivity of the urethane groups of the molecule, the new monomer is seen to be an alternative to BisGMA .

The aim of this study was to compare two nano-hybrid RBCs composed of new matrix monomers to conventional nano-hybrid RBCs available on the market, with regard to macro-mechanical properties, such as flexural strength and flexural modulus, and micromechanical measurements of the modulus of elasticity ( E micro ). Furthermore the focus was laid on the behavior of nano-hybrid RBCs after aging and storage in three different solutions: distilled water, artificial saliva and a 1:1 alcohol–water mixture for 28 days, compared to 24 h water storage. Following null hypotheses were tested: nano-hybrid RBCs composed of new monomers, such as TCD-urethane or dimer acid based methacrylates, show no differences in the mechanical properties when compared to nano-hybrid RBCs using conventional BisGMA monomers. The behavior of all six measured materials is similar in the three storing conditions.

Materials and methods

The six measured materials were nano-hybrid RBCs. Material composition, lot numbers, colors and manufacturers are shown in Table 1 .

Table 1
Materials, manufacturers, chemical composition of the matrix and the filler and filler content by weight and volume % (content (w/v)).
Material Manufacturer Color Lot-no. Matrix Filler Content (w/v)
Grandio Voco A 3 0921103 BisGMA, UDMA, di-methacrylate, TEGDMA Fluorosilicate glass, SiO 2 87/71.4
N’Durance Septodont A 3 G-9020-11 BisGMA, UDMA, dicarbamate dimethacrylate dimer acid Ytterbium-fluorid, bariumglass, quarz 80/65
Venus Diamond Heraeus A 3 010029 TCD-DI-HEA, UDMA Barium-aluminum-fluoride-glass 81/64
Miris 2 Coltène/Whaledent S 2 0191818 Methacrylates Bariumglass, SiO 2 80/65
Premise Kerr A 3 3120178 TEGDMA, BisGMA Bariumglass, SiO 2, pre-polymerized fillers 84/71.2
Simile Jeneric Pentron A 3 190633 PCBisGMA, BisGMA, UDMA, HDDMA Barium silicate-glass, circonium-silicate, SiO 2 75/66
Data are provided by the manufacturers.
BisGMA: bisphenol A diglycidyl methacrylate; UDMA: urethane dimethacrylate; TEGDMA: triethylene glycol dimethacrylate; TCD-DI-HEA: 2-propenoic acid; (octahydro-4,7-methano-1H-indene-5-diyl) bis(methyleneiminocarbonyloxy-2,1-ethanediyl) ester; PCBisGMA: pentron clinical-bisphenol A diglycidyl methacrylate; HDDMA: hexanedioldimethacrylate.

Flexural strength and flexural modulus were determined in a three-point-bending test, using a universal testing machine (Zwick/Roell Z 2.5, AST. GmbH, Ulm, Germany). The bar- shaped specimen, measuring 2 mm × 2 mm × 16 mm, were produced by applying the composites to a stainless steel mold, and were then shaped between two parallel glass plates, covered with transparent matrix strips prior to light curing. Irradiation occurred on top and bottom of the specimens, with three light exposures of 20 s per side, overlapping one irradiated section no more than 1 mm of the diameter of the light guide (Elipar™ Freelight 2, 3M ESPE), in order to prevent multiple polymerization. After removal from the mold the specimens were grinded with silicone carbide paper (grit size P 1200/4000 (Leco)) in order to get rid of disturbing edges or bulges. 80 such specimens of each material were produced and then stored in distilled water for 24 h at 37 °C, 20 of them being loaded to fracture right after the 24 h water storage. The remaining 60 samples were thermocycled for 5000 cycles at 5–55 °C, before being randomly divided into groups of 20 specimens, stored in either distilled water, commercial artificial saliva (see Table 2 for composition) or a 50:50 mixture of 96% ethanol and distilled water for four weeks. The storage solutions were exchanged daily, keeping the volume of liquid constant. During testing all samples were immersed in distilled water at room temperature, and were loaded until failure at a crosshead speed of 0.5 mm/min, the distance between the supports of the three-point testing device being 12 mm. A force-deflection diagram was recorded during bending by using a strain gage to measure the deformation of the beam. The flexural modulus was calculated from the slope of the linear part of this graph.

Table 2
Composition of the artificial saliva.
pH 6.9
Composition of 1000 ml
Potassium chloride 1.20 g
Sodium chloride 0.84 g
Potassium phosphate 0.26 g
Calcium-chloride-dihydrate 0.14 g

The modulus of elasticity was also determined on microscopic scale ( E micro ), using the fragments of the three-point-bending test-specimens. Six such fragments were used for each measurement, recording 10 measuring points on each sample with an automatic microhardness indenter (Fischerscope H100C, Fischer, Germany according to DIN 50359-1:1997-10) . Before measuring the surfaces of the samples were polished with silicone carbide paper (grit size P 1200/4000 (Leco)) and 1 μm diamond spray. The test procedure was carried out force controlled, where the increase and decrease of the test load happened at a constant speed between 0.4 and 500 mN. The load and the penetration depth of the indenter were recorded continuously during the load–unload hysteresis. The indentation modulus ( E micro ), which can be compared to the modulus of elasticity, could be detected from the slope of the tangent of the indentation depth curve during unloading at the maximum force of 500 mN.

Statistical analysis

Data were statistically analyzed by using one-way ANOVA and Tukey HSD post hoc test ( α = 0.05), allowing a comparison of the results within each material and among all materials and storage conditions. Also a Pearson’s correlation analysis was performed (SPSS Statistics 17, Chicago, IL). The influence of the parameters filler volume, filler weight, storage solution and material were deduced from the information of a multivariate analysis (general linear model). Additionally, for the evaluation of the flexural strength data, a Weibull analysis was carried out in order to obtain the parameters m , Weibull modulus, expressing the variation in the distribution of strength values, and σ 0 , the characteristic strength, representing the stress that causes 63.2% (= F , the failure probability) of the samples to fail.

Materials and methods

The six measured materials were nano-hybrid RBCs. Material composition, lot numbers, colors and manufacturers are shown in Table 1 .

Table 1
Materials, manufacturers, chemical composition of the matrix and the filler and filler content by weight and volume % (content (w/v)).
Material Manufacturer Color Lot-no. Matrix Filler Content (w/v)
Grandio Voco A 3 0921103 BisGMA, UDMA, di-methacrylate, TEGDMA Fluorosilicate glass, SiO 2 87/71.4
N’Durance Septodont A 3 G-9020-11 BisGMA, UDMA, dicarbamate dimethacrylate dimer acid Ytterbium-fluorid, bariumglass, quarz 80/65
Venus Diamond Heraeus A 3 010029 TCD-DI-HEA, UDMA Barium-aluminum-fluoride-glass 81/64
Miris 2 Coltène/Whaledent S 2 0191818 Methacrylates Bariumglass, SiO 2 80/65
Premise Kerr A 3 3120178 TEGDMA, BisGMA Bariumglass, SiO 2, pre-polymerized fillers 84/71.2
Simile Jeneric Pentron A 3 190633 PCBisGMA, BisGMA, UDMA, HDDMA Barium silicate-glass, circonium-silicate, SiO 2 75/66
Data are provided by the manufacturers.
BisGMA: bisphenol A diglycidyl methacrylate; UDMA: urethane dimethacrylate; TEGDMA: triethylene glycol dimethacrylate; TCD-DI-HEA: 2-propenoic acid; (octahydro-4,7-methano-1H-indene-5-diyl) bis(methyleneiminocarbonyloxy-2,1-ethanediyl) ester; PCBisGMA: pentron clinical-bisphenol A diglycidyl methacrylate; HDDMA: hexanedioldimethacrylate.

Flexural strength and flexural modulus were determined in a three-point-bending test, using a universal testing machine (Zwick/Roell Z 2.5, AST. GmbH, Ulm, Germany). The bar- shaped specimen, measuring 2 mm × 2 mm × 16 mm, were produced by applying the composites to a stainless steel mold, and were then shaped between two parallel glass plates, covered with transparent matrix strips prior to light curing. Irradiation occurred on top and bottom of the specimens, with three light exposures of 20 s per side, overlapping one irradiated section no more than 1 mm of the diameter of the light guide (Elipar™ Freelight 2, 3M ESPE), in order to prevent multiple polymerization. After removal from the mold the specimens were grinded with silicone carbide paper (grit size P 1200/4000 (Leco)) in order to get rid of disturbing edges or bulges. 80 such specimens of each material were produced and then stored in distilled water for 24 h at 37 °C, 20 of them being loaded to fracture right after the 24 h water storage. The remaining 60 samples were thermocycled for 5000 cycles at 5–55 °C, before being randomly divided into groups of 20 specimens, stored in either distilled water, commercial artificial saliva (see Table 2 for composition) or a 50:50 mixture of 96% ethanol and distilled water for four weeks. The storage solutions were exchanged daily, keeping the volume of liquid constant. During testing all samples were immersed in distilled water at room temperature, and were loaded until failure at a crosshead speed of 0.5 mm/min, the distance between the supports of the three-point testing device being 12 mm. A force-deflection diagram was recorded during bending by using a strain gage to measure the deformation of the beam. The flexural modulus was calculated from the slope of the linear part of this graph.

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on The mechanical stability of nano-hybrid composites with new methacrylate monomers for matrix compositions
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