Interfacial integrity of low-shrinkage composite restorations related to the filling technique and consistency

Purpose: To evaluate the influence of filling techniques and consistency of low-shrinkage composites on the bond strength and internal gap formation in Class I bonded restorations. Two low-shrinkage restorative systems (N’Durance/Septodont and Filtek Silorane/3M ESPE) were used to restore Class I preparations. Bulk increment, two oblique increments or two horizontal increments were performed and Filtek Z350XT was used as a control. Each increment was photoactivated for 40?s and the restored teeth were then stored for 24?h at 37?°C before being longitudinally sectioned and the internal margins of the restorations dyed with 1% acid-red propylene glycol solution for 20?s. The specimens were water rinsed and digital images were recorded for the internal gap formation (%) analysis. Stick-shaped specimens were also tested in tension to evaluate the bond strength. Comparative consistency of unset composites was also performed. N’Durance exhibited the highest incidence of internal gaps amongst all the filling techniques. Filtek Silorane showed no significant difference of internal gap formation in comparison with Filtek Z350XT, regardless of the filling technique. Filtek Z350XT exhibited the highest bond strength and N’Durance the lowest when horizontally applied; bond strength of Filtek Silorane was not influenced by the filling technique. Both Filtek Z350XT and N’Durance presented lower consistency. Low-shrinkage composites seem to present equivalent or inferior bond strength and internal gap formation when compared to the methacrylate composite. The quality of the Class I preparation is dependent on the material and filling techniques.     

Effect of resin type and content on properties of composite particleboard made of a mixture of wood and rice husk

This study investigated the effect of resin type and content on the dimensional stability and mechanical properties of single-layer composite particleboards made of a mixture of wood particles (70 wt%) and rice husk particles (30 wt%). Two types of resin, urea–formaldehyde (UF) resin and phenol–formaldehyde (PF) resin, were used in the experiments at three different contents which were 8, 10, or 12 wt%. The dimensional stability of the samples was significantly improved by increasing the resin content. When the contents of the UF and PF resins increased from 8 to 12 wt%, the WA values of the samples decreased to18% and 33%, respectively. Similar results were also observed for the TS values. The UF resin bonded samples swelled two times more than the PF resin bonded particleboard. The mechanical properties of the PF resin bonded samples were better than the UF resin bonded samples. When the contents of the UF and PF resins increased from 8% to 12 wt%, the internal bond strength values of the samples increased to 21% and 41%, respectively. The bending strength and modulus of elasticity of the samples were not significantly increased by increasing contents of the UF and PF resins, except for the 12 wt% content.     

Effect of proanthocyanidin incorporation into dental adhesive on durability of resin–dentin bond

BackgroundProanthocyanidin has shown to have beneficial effects on dentin bonding via its collagen cross-linking and protease inhibitory effects.ObjectiveThis study evaluated the effect of incorporation of 1–3% PA into a dental adhesive on durability of resin–dentin bond.Materials and methodsThe experimental adhesive was first formulated by combining 50 wt% comonomer mixtures with 50 wt% ethanol. PA was then added to the ethanol-solvated adhesive to yield three groups of adhesives at concentrations of 1.0 wt%, 2.0 wt% and 3.0 wt%. The PA-free adhesive served as control. Flat dentin surfaces from forty extracted third molars were etched with 32% phosphoric acid and the specimens were randomly assigned to one of the four adhesive groups. Two layers of experimental adhesives were applied to etched dentin and light-cured for 20 s after solvent evaporation. Composite build-ups were performed using Filtek Z250 (3M ESPE). The bonded teeth were divided into three subgroups for different methods of storage: (1) 24 h indirect water exposure (IE), (2) 6 M IE and (3) 6 M direct water exposure (DE). After the designated period of water storage, the bonded teeth were sectioned into 0.9 mm×0.9 mm beams for bond strength testing. Bond strength data were evaluated by two-way ANOVA and Tukey׳s tests (α=0.05). Interfacial nanoleakage was examined using a field-emission scanning electron microscopy. Two-way ANOVA and Tukey׳s tests were used to examine the effects of PA concentration and water exposure on bond strength and percentage of nanoleakage (α=0.05).ResultsTwo-way ANOVA showed that the factors, water exposure and PA concentration had a significant effect on bond strength (p<0.001). Interaction between the two factors was also significant (p<0.001). Bond strength of all four adhesives decreased with PA concentrations and ageing. Type of water exposure had no effect on the bond strength of PA-incorporated adhesive; while direct water exposure significantly reduced the bond strength of PA-free adhesive. Conversely, the factors, water exposure and PA concentration showed a significant effect on nanoleakage percentage (p<0.001). Interaction between the two factors was not significant (p>0.05).ConclusionIncorporation of proanthocyanidin into a dental adhesive did not prevent resin–dentin bond degradation over time.     

Compatibility of silorane composite repaired with methacrylate-based restorative systems

The aim of this study was to evaluate the repair bond strength of silorane composites using either the silorane or methacrylate-based restorative systems. Expired silorane composite was used as the substrate material in all experimental groups. Silorane blocks (5?×?5?×?4?mm) were fabricated and stored at 37?°C for 24?h. Six experimental groups were developed according to the repair: I-silorane composite (no intermediary); II-P90 Bond/Silorane; III-P90 Adhesive System (primer/bond)/silorane; IV-P90 bond/Scotchbond Universal/methacrylate composite (Filtek P60); V-Scotchbond Universal/methacrylate; and VI-silane/Adper Single Bond 2/methacrylate. The repaired blocks were stored for 24?h at 37?°C, and then sectioned, yielding stick-shaped specimens (0.5?mm²) that were tested in tensile (0.5?mm/min). The results were analyzed using ANOVA/Tukey test (α?=?0.05). The interfacial micromorphology and nanoleakage were also analyzed under SEM. Scotchbond Universal/methacrylate composite, either associated with the P90 bond or not, exhibited similar bond strength to that of P90 Adhesive System/silorane composite. Scotchbond Universal either associated with the P90 Bond or not to repair the silorane allowed no pre-testing failures. The worst scenarios were repairing the silorane with no intermediary (G-I) or combination silane/Adper Single Bond 2/methacrylate composite (G-VI) that presented significantly lower bond strengths and higher incidences of pre-failure testing. The importance of the silane was not confirmed. Characteristic micromorphology and no signs of nanoleakage were identified in all experimental groups. The silane-containing, phosphorylated methacrylate-based adhesive associated with a methacrylate composite was proven to reliably repair the expired composite in a similar way to that of the application of dedicated silorane adhesive.     

Effect of methyl methacrylate monomer on bond strength of denture base resin to acrylic teeth

Bond failures at the acrylic teeth and denture base resin interface are still a common clinical problem in prosthodontics. The effect of methyl methacrylate (MMA) monomer on the bond strength of three types of denture base resins (Acron MC, Lucitone 550 and QC-20) to two types of acrylic teeth (Biotone and Trilux) was evaluated. Twenty specimens were produced for each denture base resin/acrylic tooth combination and were randomly divided into control (acrylic teeth received no surface treatment) and experimental groups (MMA was applied to the surface of the acrylic teeth for 180 s) and were submitted to shear tests (1 mm/min). Data (MPa) were analyzed using three-way ANOVA/Student's test (α=0.05). MMA increased the bond strength of Lucitone denture base resins and decreased the bond strength of QC-20. No difference was detected for the bond strength of Acron MC base resin after treatment with MMA.     

Bond strength and chemical interaction of self-adhesive resin cements according to the dentin region

The aim of the study is to evaluate the effectiveness and level of chemical interaction of self-adhesive resin cements (SRCs) according to the dentin region. One hundred eight sound human third molars and three SRCs were selected: Bifix SE (Voco), Maxcem Elite (Kerr), and RelyX U200 (3M ESPE). Ninety human molars were used for the bond strength test and 18 teeth for the X-ray diffraction (XRD) characterization. A flat surface of superficial, deep, or axial dentin was exposed. For bond strength evaluation, 90 indirect composite resin restorations (10 mm in diameter, 2.0 mm-thick) were built and cemented with one of the SRCs according to the manufacturer's instructions. The restored teeth were then cut into sticks with cross-sectional areas of 0.8 mm² and tested in tensile at a speed of 0.5 mm/min (n=10). The results of bond strength were statistically analyzed by two-way ANOVA and Tukey's test (α=0.05). The fractured specimens were classified under SEM. The remaining teeth were further sectioned in order to build dentin fragments with 2.0 mm² of area and 0.2 mm in thickness for XRD analysis. In general, significantly higher bond strength was found when bonding to axial and deep dentin compared to superficial dentin. Comparing the bonding effectiveness of the SRCs, taking into account the mean bond strength obtained in the 3 dentin regions, the study found no significant difference (p>0.05). Although RelyX U200 showed similar bond strength irrespective of the dentin region (p>0.05), the bonding results of the other 2 SRCs varied significantly (p<0.05). There was a higher incidence of cohesive failure in the SRCs for all groups. The XRD analysis detected different perceptual reductions of hydroxyapatite crystallinity for all SRCs, indicating a particular chemical interaction in each experimental condition. Thus, it can be concluded that the bond strength and chemical interaction of the SRCs can vary significantly according to the dentin region.     

Effect of different adhesive protocols on bond strength between composite resins for indirect use and repair materials

Abstract

The aim of this work is to evaluate the effect of different adhesive protocols on the bond strength (SBS) of composite resin for indirect use to repairs of bulk-fill or conventional nanoparticulated composites. Forty-eight cylindrical specimens of composite resin for indirect use were prepared, aged, and randomly divided into four groups (n?=?12), a control group without any adhesion protocol, and three experimental groups: Silane?+?Scotch Bond Multipurpose adhesive (S?+?SBMP), Tetric N Bond Universal (TBU), and Single Bond Universal (SBU). The treated surfaces were restored using two different composite resins: Filtek Bulk-Fill or Filtek Z350XT. Then, the specimens were submitted to the SBS test, and the resultant data were analyzed with ANOVA on ranks test and Tukey’s test (α?=?0.05). There were no significant differences between the two types of resins used as repair material. For both resins, the groups treated with S?+?SBMP obtained the highest values (p?<?0.001). Groups TBU and SBU did not have statistically significant differences between them. Pre-treatment with a silane coupling agent and a layer of a hydrophobic adhesive can improve the bond strength of repairs performed on a composite resin for indirect use.     

Heat treatment of silanized feldspathic ceramic: Effect on the bond strength to resin after thermocycling

PurposeThis study aimed to evaluate the effect of heat treatment (at 77 °C) of a silanized feldspathic ceramic on microtensile bond strength (μTBS) with a resin cement before and after being aged by thermocycling.Material and methodsTwenty-four blocks (12×10×4 mm³) of a CAD/CAM feldspathic ceramic (Vitablocks Mark II, Vita) were obtained and randomly divided into three groups, according to the surface treatment prior to the cementation: Group AS – hydrofluoric acid 10%+silane; Group S77 – silane+heating at 77 °C for 60 s; and Group AS77 – hydrofluoric acid 10%+silane+heating at 77 °C for 60 s. Ceramic blocks were cemented to composite resin blocks with a resin cement. The sets were subsequently cross-sectioned into 1 mm² beams for μTBS testing. The beams of each group were randomly divided into two subgroups: aging (thermocycling, 12,000 cycles between 5 °C and 55 °C) and non-aging (tested immediately). One-way ANOVA and Tukey's test (α=0.05) and Weibull analysis (95% CI) were used to analyze the data.ResultsGroup AS77 had the lowest pre-test failure number during the cutting among the groups. There was no significant difference (p=0.255) between the μTBS mean values of the non-aged groups. After aging, the mean value of S77 was significantly lower than those of AS77 and AS (p=0.005). There was no difference in the Weibull modulus (m) and characteristic strength (σ₀) of the aged and non-aged groups for all comparisons. Before aging, heat treatment of silanized feldspathic ceramic (non acid-etched surface) demonstrated bond strength similar to that achieved with hydrofluoric-acid-etching treatment however, it had lower bond strength after aging.ConclusionThe combination of hydrofluoric-acid-etching treatment with heat treatment silanized feldspathic ceramic did not improve the bond strength of the interface.     

Effect of erosive challenge and Nd:YAG laser irradiation on bond strength of adhesive systems to dentin

PurposeTo evaluate the effect of Nd:YAG laser irradiation and erosive challenge on bond strength of two adhesive systems to dentin.MethodsTwenty bovine incisors were cut and grounded to obtain eighty slabs of flat dentin. Specimens were allocated into eight groups, based on: adhesive system—a two-step etch-and-rinse and a two-step self-etch; laser irradiation—Nd:YAG (1 W/10 Hz) or control (no laser irradiation); and erosive challenge after restorative procedure—presence or absence of erosive challenge. Nd:YAG laser groups were submitted to laser irradiation before the restorative procedure. Blocks of composite resin were built up on the bonded surfaces with a Southern Dental Industries device to perform shear bond strength (SBS) test. After, each specimen of erosive challenge, groups were subjected to immersion in Sprite Zero^® (20 ml/2 h/24 °C/under agitation). The SBS test (0.5 mm/min) was performed after 24 h of water storage at 37 °C. Failure mode was evaluated with a stereomicroscope (X400). Data were analyzed with three-way ANOVA and Tukey’s post hoc tests (α=0.05).ResultsThe etch-and-rinse adhesive system presented higher bond strength values than self-etch adhesive. Laser irradiation increased the bond strengths values when erosive challenge was present. The predominant failure mode observed was adhesive.ConclusionsThe irradiation of Nd:YAG laser positively influences the bond strength values when erosive challenges are present. Moreover, the etch-and-rinse adhesive system is a better option to be used in dentin in this clinical condition.     

Influence of storage time on bond strength of self-etching adhesive systems to artificially demineralized dentin after a papain gel chemical–mechanical agent application

ObjectivesThe aim of this study was to evaluate the influence of storage time on the bond strength of self-etching adhesive systems to artificially demineralized dentin submitted to application of a papain-based chemical mechanical agent for carious tissue removal.Materials and MethodsTwenty-four blocks of human coronal dentin were randomly divided into 2 groups: (1) restored with a two-step self-etching adhesive system (Clearfil SE Bond); (2) restored with a one-step self-etching adhesive system (One-Up Bond F Plus). After artificial caries induction, the specimens were treated with papain-based gel, received an application of the adhesive system according to the respective group, and blocks of microhybrid resin composite measuring 5.0 mm high and 5.0 mm wide were fabricated incrementally on the tooth, which would later be cut to obtain sticks with a bond area of around 1 mm², for use in the microtensile tests. After this, half of the sticks obtained from each tooth were submitted to the microtensile test 24 h later, while the other half were stored in water, in an oven at 37° C for a period of 180 d. The specimens were submitted to the microtensile test in a Universal Test Machine at a speed of 0.5 mm/min. The surfaces of the fractured test specimens were examined visually under a stereoscopic loupe in order to classify the fracture type. After exploratory analysis of the data, the two-way Analysis of Variance (ANOVA) and the Tukey test were applied. The data on the fracture types were analyzed by the Exact Fisher test.ResultsNo statistically relevant differences were verified among the means of the microtensile bond strength values of the adhesive systems evaluated in the different storage times. Mixed and adhesive fractures types were observed for both groups.ConclusionStorage time did not influence the bond strength of the two-step or one-step self-etching adhesive systems to artificially demineralized dentin submitted to application of a papain-based chemical mechanical agent for carious tissue removal. Both adhesive systems provided similar bond strength at different storage times.     

Effect of pH and application times of a meta-phosphoric acid on resin-dentin bonding properties

PurposeTo evaluate the effect of different pHs of 40% meta phosphoric acid (MPA) in comparison with a conventional 37% ortho-phosphoric acid under different application times in terms of resin-dentin bond strength (μTBS), nanoleakage (NL) and the dentin-etching pattern.Materials and methodsNinety-six molars were randomly assigned into 12 experimental conditions according to the combination of the independent variables: Acid [37% ortho-phosphoric acid (OPA), 40% meta-phosphoric acid (MPA) in the following pHs – 0.5, 1 and 2] and Application Time [7, 15 and 30 s]. After restorations, specimens were sectioned into resin–dentine beams (0.8 mm²) tested under tension (0.5 mm/min). Selected beams from each tooth were used for NL evaluation. The dentin-etching pattern of the OPA and MPA was examined under a scanning electron microscop. Acid vs application time data were subjected to two-way ANOVA and Tukey's test (α=0.05)ResultsMPA pH 0.5 showed μTBS means similar to OPA, independently the application time (p>0.05). A statistically higher NL was observed for OPA in comparison with MPA, regardless the application time (p<0.03). When MPA pH 0.5 was applied at the different times, the demineralised substrate showed presence of minerals along dentin surface.ConclusionThe use of 40% meta-phosphoric acid with pH 0.5 is an alternative acid-etching agent with similar immediate bond strength and lower nanoleakage than the traditional 37% ortho-phosphoric acid.     

Bond strength and adhesive interface analysis using EDTA as a dentin conditioner

This study evaluated dentin bond strength and dentin-resin interfacial morphology using phosphoric acid-etching and EDTA-conditioning. The occlusal enamel of twenty-four human third molars was removed to expose the dentin surfaces, which were polished with a SiC paper (600 grit). Teeth were randomly divided into three groups (n = 8): etching with 37% phosphoric acid (15 s), conditioning with 0.1 M EDTA (60 s) and 0.5 M EDTA (120 s). Adhesive (XP Bond, Dentsply) was applied and three layers (2 mm each) of composite (EsthetX, Dentsply) were placed and light-activated separately (20 s). Teeth were sectioned to obtain specimens for the microtensile bond strength test (1 mm² at cross section). Half of the specimens was tested in a universal testing machine (EZ Test, Shimadzu) after 24 h and the other half after storage for 10 months. Failure mode and adhesive interface were analyzed using scanning electron microscopy. Data were analyzed by mixed models for repeated measures (PROC MIXED) and Tukey-Kramer test, considering a significance level of 5%. Mean bond strength values (SD) after 24 h and 10 months were, respectively (in MPa): phosphoric acid: 37.3 (7.7) / 33.9 (6.7); 0.1 M EDTA: 14.7 (7.3) / 15.1 (10.1); 0.5 M EDTA: 25.1 (7.7) / 21.1 (14.1). Dentin treatment with EDTA and phosphoric acid resulted in hybrid layer and resin tags formation. Mixed failures were prevalent for all groups tested after 24 h. Storage for 10 months changed the failure mode for the 0.5 M EDTA group to adhesive failure (between dentin and bonding agent). EDTA applied for 60 s yielded lower bond strength results compared to phosphoric acid and EDTA (120 s), but there was no significant reduction after 10 months of storage for any of the tested groups.     

Preparation of poly (ethylene glycol) acrylate grafted polystyrene resin for solid-phase peptide synthesis

The solid-phase synthesis resin with high loading capacity was prepared through grafting poly (ethylene glycol) acrylate monomer from Merrifield resins via activators generated by electron transfer atom transfer radical polymerization. The grafted resins demonstrate well-swellability in both polar and nonpolar solvent such as dichloromethane, dimethylformamide, ethanol, tetrahydrofuran, acetonitrile, methanol and water. Particularly, the swelling ability of the grafted resin has reached two-fold of Merrifield resin in the polar solvent such as acetonitrile, methanol and water, and it enable high functional loadings up to 0.5–1.2 mmol g⁻¹ compared with the conventional polystyrene-grafting-poly (ethylene glycol) (0.15–0.25 mmol g⁻¹). This resin was derived to be used for synthesis of a difficult sequence-acyl carrier protein fragment 65-74 (ACP 65-74). The quantity and purity of peptide obtained from the grafted resin were higher than when the commercial Wang resin was used. The synthesis efficiency enhanced with the increase of grafting chains’ length within the range of hydroxyl capacity at 0.5–1.0 mmol g⁻¹. It was relative that the longer grafting chains were favor to suppress the hydrophobicity of the Merrifield resin.     

Simultaneous improvements in the cryogenic tensile strength,ductility and impact strength of epoxy resins by a hyperbranched polymer

Improvements in mechanical properties at low temperatures are desirable for epoxy resins such as diglycidyl ether of bisphenol A (DGEBA) that are often used in cryogenic engineering applications. In this study, a hydroxyl functionalized hyperbranched polymer (H30) is employed to improve the mechanical properties of a DGEBA epoxy resin at liquid nitrogen temperature (77 K). The results show that the tensile strength, failure strain (ductility) and impact strength at 77 K are simultaneously improved by adding a proper content of H30. The maximum tensile strength at 77 K is increased by 17.7% from 98.2 MPa of pure epoxy resin to 115.6 MPa of modified epoxy system for the 10 wt% H30 content. The failure strain at 77 K increases consistently with the increase of H30 content. The maximum impact strength at 77 K is attained by introduction of 10 wt% H30 with an improvement of 26.3% over that of pure epoxy resin. For the purpose of comparison, the mechanical properties of modified epoxy resins at room temperature (RT) are also investigated. It is interesting to note that the impact strength is not lower at 77 K than that at RT for the modified systems. Moreover, the glass transition temperature (T_g) is not reduced by the addition of H30 in appropriate amounts.     

Bonding ZrB2-SiC-G ceramics using modified organic adhesive for engineering applications at ultra high temperatures in air

It is a challenge to bond ceramics for engineering applications at ultrahigh temperatures in air. In this paper, a high temperature organic adhesive (HTOA) was prepared using methylphenylsilicone resins (MPSR) as the matrix, trisilanolisobutyl-methylsilicone resin/polyhedral oligomeric silsesquioxane (POSS) as the modifier, ZrB₂, SiO₂ and Si powders as the inorganic fillers, and γ-aminopropyltriethoxysilane (KH550) as the curing agent. The synthesized HTOA was used to bond ZrB₂-SiC-G ceramic (ZSG). The ceramic yield of MPSR was increased from 71% to 91% after being modified by trisilanolisobutyl-POSS. The average shear strength of ZSG joints bonded by HTOA was 13.2 MPa at room temperature. After 1500 °C/1 h processing, the bonding strength between HTOA and ZSG ceramic was 53.8 MPa. The inter-diffusion of elements between the HTOA and the ZSG occurred at 1500 °C and ZrSiO₄ compound was formed via the interface reaction. The excellent high-temperature performance of the prepared HTOA makes it one of the convenient and effective organic adhesive for joining ZSG for engineering applications at ultrahigh temperatures in air.     

Effects of simulated pulpal pressure,mechanical and thermocycling challenge on the microtensile bond strength of resin luting cements

This study aimed at comparing the microtensile bond strength (µTBS) of three simplified luting strategies after different aging processes. Sixty human molars were prepared to expose flat middle dentin surfaces which received the following luting procedures: (i) SB+ARC – two-step etch-and-rinse adhesive+conventional resin cement (Adper Singlebond 2+RelyX ARC, 3M-ESPE); (ii) S3+PAN – one-step self-etch adhesive+conventional resin cement (Clearfil S3+Panavia F2.0, Kuraray Medical); (iii) U200 – self-adhesive resin cement (RelyX U200,3M-ESPE). The specimens were finally restored by indirect resin composite procedures (Filtek Z100,3M-ESPE). The aging regimens were water storage at 37 °C for one week (control), one week of 20 cm H₂O simulated pulpal pressure (SPP), 200,000 mechanical loading (ML) cycles, or 5000 thermal cycles (TC). The µTBS data was analyzed by two-way ANOVA and Tukey's test (α=0.05). SB+ARC showed significantly higher µTBS for control and all aging processes (p<0.001). Nevertheless, TC had no effect on the bond strength of SB+ARC. No difference in µTBS was observed between S3+PAN and U200 after SPP (p=0.251), but significant lower values were found for U200 after ML (p=0.010) besides being superior in the control groups (p<0.001). For U200, all ageing regimens induced significant reductions in the bond strength (p<0.001) with a more pronounced negative effect after ML. S3+PAN showed significant lower bond strength (p=0.010) only after ML aging. Two-step etch-and-rinse adhesive associated with dual-curing conventional resin cement may present the highest overall µTBS. However, the use of S3 one-step self-etch adhesive along with conventional resin cements may provide the most stable luting performance under the tested aging strategy.     

Synthesis and characterization of cross-linked polymeric nanoparticles and their composites for reinforcement of photocurable dental resin

Reactive polymeric nanoparticles were formed for reinforcement of photocurable dental resin. Cross-linked polymeric nanoparticles were synthesized by emulsion polymerization of mono- (methyl methacrylate; MMA) and trifunctional (trimethylol propane trimethacrylate; TMPTMA) monomers. The nanoparticles were dispersed in bisphenol A glycol dimethacrylate (Bis-GMA) based dental resin matrix in the range of 5–25 wt% to form photocurable nanocomposites.The effect of reactive polymeric particles on the mechanical properties of photocurable dental resin was investigated. Polymerization shrinkage, polymerization shrinkage stress, viscosity, diametral tensile strength, compressive, and flexural strength of the nanocomposites have been studied.It was observed that the cross-linked nanoparticles significantly influenced the mechanical properties of the reinforced dental resin nanocomposites.     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

Preparation and mechanical properties of modified epoxy resins with flexible diamines

Diglycidyl ether of bisphenol A (DGEBA) is one of the most widely used epoxy resins for many industrial applications, including cryogenic engineering. In this paper, diethyl toluene diamine (DETD) cured DGEBA epoxy resin has been modified by two flexible diamines (D-230 and D-400). The cryogenic mechanical behaviors of the modified epoxy resins are studied in terms of the tensile properties and Charpy impact strength at cryogenic temperature (77 K) and compared to their corresponding properties at room temperature (RT). The results show that the addition of flexible diamines generally improves the elongation at break and impact strength at both RT and 77 K. The exception is the impact strength at 77 K filled with 21 wt% and 49 wt% D-400. Further, two interesting observations are made: (a) the cryogenic tensile strength increases with increasing the flexible diamine content; and (b) the RT tensile strength can only be improved by adding a proper content of flexible diamines. It is concluded that the addition of a selected amount namely 21–78 wt% of D-230 can simultaneously strengthen and toughen DGEBA epoxy resins at both RT and 77 K. However, only the addition of 21 wt% D-400 can simultaneously enhance the strength and ductility/impact strength of DGEBA epoxy resins at RT. The impact fracture surfaces are examined using scanning electron microscopy (SEM) to explain the impact strength results. Finally, differential scanning calorimetry (DSC) analysis shows that the glass transition temperature (T_g) decreases with increasing the flexible diamine content. The presence of a single T_g reveals that the flexible diamine-modified epoxy resins have a homogeneous phase structure.     

Bond strength evaluation of composite resin bonded to glass ionomer cements after different periods of setting

This study investigated the time elapsed after setting of glass ionomer cements on the bond strength to composite resin restorations. Bovine incisors received cavity preparations on the buccal surface (6 mm×6 mm×2 mm) and the specimens were tested according to cement type (conventional and resin-modified) and time elapsed before performing the restorations: GC10m: conventional glass ionomer cement and 10 min time elapsed after setting; GC24h: conventional cement and 24 h after setting; GC7d: conventional cement and 7 days after setting; GRM10m: resin-modified glass ionomer cement and 10 min after setting; GRM24h: resin-modified cement and 24 h after setting; and GRM7d: resin-modified cement and 7 days after setting. Specimens were subjected to micro-shear testing and the data were analyzed by Analysis of Variance and Tukey′s test (p=0.05). Bond strength of restorations performed on conventional cement after 10 min of time elapsed presented the lowest mean values and differed statistically from values at 24 h and 7 days. Resin-modified cement after 24 h presented the highest mean values and differed statistically from values at 10 min and 7 days. The time elapsed after setting of glass ionomer cement may interfere in the bond strength to composite restorations.