Properties of eight methacrylated beta-cyclodextrin composite formulations.

OBJECTIVES: Methacrylated beta-cyclodextrins (MCDs) are novel candidate dental monomers if all or some of the hydroxyl groups of beta-cyclodextrin are substituted with methacrylate groups. The main objective of this study was to evaluate mechanical properties of a number of composite formulations having MCDs as novel dental comonomers. The properties determined were flexural strength (FS), volumetric shrinkage (VS), and degree of conversion (DC). METHODS: A mass fraction of 50% of MCD monomers was mixed with a mass fraction of 50% each of a series of dimethacrylate or monomethacrylate diluent comonomers to produce consistent formulations of a workable viscosity. For comparison a resin mixture of a mass fraction of 50% Bis-GMA and a mass fraction of 50% triethyleneglycol dimethacrylate (a typical dental resin mixture) was also studied. The mixtures were activated with camphorquinone and ethyl 4-dimethylamino benzoate. One part by mass of each activated resin formulation was mixed with three parts by mass of glass filler. Samples for the FS tests were prepared in (2 x 2 x 25) mm3 molds by light-curing the composites for 2 min on each side. The cured samples were immersed in 37 degrees C water for 24 h, and FS was measured with an Instron machine at a crosshead speed of 0.5 mm/min. VS was measured by a computer-controlled mercury dilatometer. DC was measured by near-infrared spectroscopy. RESULTS: The properties of the MCD-based composites depended on the kind of diluent used. With these MCD monomers, diluents of triethyleneglycol dimethacrylate, 1,10-decamethylenediol dimethacrylate, or benzyl methacrylate yielded the best composite properties. SIGNIFICANCE: Although not yet fully optimized, MCD-based composite formulations containing triethyleneglycol dimethacrylate, 1,10-decamethylenediol dimethacrylate, or benzyl methacrylate yielded flexural strength and volumetric shrinkage values were comparable to those of the Bis-GMA/triethyleneglycol dimethacrylate controls. These findings lend support for further development and evaluations of polymerizable cyclodextrin derivatives for use in dental materials.     

Effects of water content and initiator composition on photopolymerization of a model BisGMA/HEMA resin.

AIMS: The purpose of this study was to evaluate the effects of photoinitiator type and water content on the polymerization rate (Rp) and degree of conversion (DC) of a model BisGMA/HEMA-based resin. MATERIALS AND METHODS: The comonomer mixture consisted of BisGMA/HEMA (60/40 by weight). Different two- or three-component photoinitiator systems were incorporated. Two-component systems were 0.5% CQ (camphorquinone) and 0.5% DMAEMA (2-(dimethylamino) ethyl methacrylate) or 0.5% CQ and 0.5% 4E (ethyl 4-dimethylaminobenzoate). The three-component systems were added 1% DPIHP (diphenyliodonium hexafluorophosphate) to the above systems. Each system was tested as made, or after addition of 5, 10, 15wt% water. When cured under a conventional dental light, the Rp and DC of each formulation was determined using time-resolved attenuated total reflection (ATR)-Fourier transform infrared (FTIR) spectroscopy. RESULTS: For mixtures containing two-component initiator systems, when the hydrophobic initiator CQ was used in combination with hydrophilic DMAEMA, Rps and DCs were dramatically decreased as a function of water content. The Rps and DCs of the hydrophobic CQ/4E system were higher than those of the CQ/DMAEMA system in the presence of water. For three-component initiator systems, incorporation of DPIHP enhanced the polymerization of all mixtures in the presence of water compared to their counterpart two-component initiators. Interestingly, the CQ/DMAEMA caused greater DC and Rp when DPIHP was used. SIGNIFICANCE: The hydrophobicity/hydrophilicity of photoinitiator components significantly affects both the DC as well as Rp when in the presence of water. The results indicate that formulation of photoinitiator components should be based on the effectiveness of the bonding systems under both dry and wet conditions.     

The effects of two monofunctional diluent monomers and two photoinitiator systems on the properties of UDMA-based composites

PURPOSE: To compare the effects of two types of monofunctional co-monomers and two types of photoinitiator systems on the properties of urethane dimethacrylate (UDMA) based dental restorative composites. METHODS: The resin blends consisted of UDMA and a diluent co-monomer at a molar ratio of 9:1. The diluent comonomers were neo-decyl vinyl ester (Neo 10) or n-hexyl methacrylate (HMA). The photoinitiator system consisted of a bis-acyl phosphine oxide (BAPO) or camphorquinone plus ethyl 4-(dimethylamino)benzoate (CQ/4E). Each initiator system was utilized at 1% or 2% by mass of the resin blend. These resin blends were mixed by hand with silanized zirconia glass (85.7% by mass) to make the various composites. Flexural strength (FS) specimens, made from these pastes by visible light photopolymerization, were tested to failure after 24 hours water storage. Mercury dilatometry was used to evaluate the shrinkage, and the degree of double bond conversion was evaluated using near infrared spectroscopy (NIR). The first hypothesis tested was that use of Neo 10 in a UDMA composite would not have an effect on properties compared to the methacrylate HMA. The second hypothesis tested was that the BAPO initiator would not have an effect on the properties of a UDMA-based composite compared to the CQ/4E initiator. RESULTS: The hypotheses were tested at alpha = 0.05 and beta = 0.20. The major finding was that the lower level (1% by mass) of BAPO resulted in lower FS, lower conversion and lower shrinkage (rejecting the hypothesis that there was no difference due to initiator used) than composites formulated with the higher level of this initiator or either level of the CQ/4E initiator system, regardless of the comonomer used. The effects of two comonomers used were not differentiated in this study with respect to the properties examined (accepting the hypothesis that there was no difference due to diluent monomer).     

Onium salt improves the polymerization kinetics in an experimental dental adhesive resin

The aim of this study was to evaluate the influence of an onium salt in the polymerization kinetics of a dental adhesive model resin. A monomer mixture, based on Bis-GMA, TEGDMA and HEMA, was used as a model dental adhesive resin, which was polymerized using camphorquinone (CQ) as a photo-initiator in addition to either binary or ternary photo-initiator systems. The binary systems were formulated with different concentrations of diphenyliodonium hexafluorphosphate or ethyl 4-dimethylaminobenzoate in relation to the monomer and 1mol% of CQ. The ternary system was a blend of 1mol% of CQ, 2mol% of ethyl 4-dimethylamino benzoate and 0.25, 0.5, 1, 2 or 4mol% of onium salt. Real time Fourier Transform infrared spectroscopy was used to investigate the polymerization reaction over the photo-activation time. When CQ was used as photo-initiator, a slow polymerization reaction was observed and a lower monomer conversion. The addition of a second component (onium salt or amine) increases the polymerization rate and conversion independent on the co-initiator concentration. Ternary photo-initiator system showed an improvement on the polymerization rate of the dental adhesive model resin leading to high conversion in short photo-activation time. Also, a mechanism for initiating polymerization using an amine or onium salt as a co-initiator component is proposed.     

Effect of proanthocyanidins and photo-initiators on photo-polymerization of a dental adhesive

ObjectivesTo evaluate the effects of proanthocyanidins (PA) and photoinitiator type on the degree of conversion (DC) and polymerization rate (PR) of a model dental adhesive.MethodsThree types of photo-initiation systems were introduced into the Bis-GMA/HEMA co-monomer mixture, resulting in four resin formulations including CQ/A (0.5 wt% CQ and EDMAB), CQ/A/I-1 (0.5 wt% CQ, EDMAB and DPIHP), CQ/A/I-2 (1.0 wt% CQ, EDMAB and DPIHP), and TPO (2.1 wt% TPO). For each resin formulation, adhesives containing 0%, 2.5%, 5% and 10% of PA with respect to the weight of resin were produced after mixing the resin with various amount of PA/ethanol solution. When light-cured, the RP and DC of each adhesive was determined using ATR-FTIR spectroscopy.ResultsAcross and within the initiator groups, the DC followed the general trend of CQ/A < CQ/A/I-1 < CQ/A/I-2 < TPO and 0-PA > 2.5-PA > 5-PA > 10-PA, respectively. The change of PR with respect to photo-initiation systems and PA content was in a similar but less pronounced pattern.ConclusionPA hampered the polymerization of all adhesives regardless of photoinitiators used. The initiator formulations CQ/A/I-2 and TPO are better fit for PA-containing adhesives, both leading to >65% DC in the presence of 5% PA.Clinical significanceThe inclusion of PA in dental adhesives has been limited by its interference with the light-curing of adhesive resins. This study found photo-initiation formulations that could maintain a satisfactory degree of monomer conversion while a significant amount of PA is incorporated.     

Apoptotic and necrotic influence of dental resin polymerization initiators in human gingival fibroblast cultures

The aim of this study was to examine the apoptotic and necrotic influence of four dental resin polymerization initiators--namely benzoyl peroxide (BPO), camphorquinone (CQ), dimethylaminoethyl methacrylate (DMAEMA), and dimethyl-para-toluidine (DMPT)--on human gingival fibroblast (HGF) cells. To this end, the growth inhibition of HGF cells with 1 mM BPO, CQ, and DMAEMA, and 500 microM DMPT was evaluated using Cell Counting Kit-8. Then, cell cycle analysis by flow cytometry was used to assess propidium iodide-stained cells (distribution of cells in G0/G1, S, G2/M phases). All four dental resin polymerization initiators induced G0/G1 cell cycle arrest. As for the patterns of cell death (necrosis and/or apoptosis), they were analyzed using Annexin V-FITC/PI staining with flow cytometry. All four dental resin polymerization initiators most likely induced necrosis.     

Influence of composition on rate of polymerization contraction of light-curing resin composites

A slow contraction may result in reduced gap formation when a restorative resin polymerizes in a dental cavity. It was the aim in the present work to investigate the rate of contraction in relation to composition of experimental light-curing resin composites. The monomer of the resin composites consisted of mixtures of BisGMA, TEGDMA, and in one series HEMA. The resins contained varying amounts of initiators, co-initiators, and inhibitor, and were made composite by adding a silanized glass filler to a content of 74% by weight of the composite paste. The polymerization contraction up to 120 sec was determined by means of the bonded-disk method. Within the ranges studied, the concentration of initiator and co-initiator in the monomer mixture had only an insignificant influence on rate of polymerization. In comparison to camphorquinone, the initiators 1-phenyl-1,2-propanedione and benzil reduced the rate of polymerization without affecting the final contraction. In comparison to N,N-dimethyl- p -aminobenzoic acid ethyl ester, N,N-cyanoethyl methylaniline was as effective, while N,N-diethanol- p -toluidine was less effective as co-initiator. A relatively high content of the inhibitor methoxyhydroquinone reduced the initial rate but not the final polymerization contraction. The rate of polymerization increased with the level of HEMA and TEGDMA in the monomer mixture. It was concluded that intrinsic slow cure may be obtained with certain compositions of resin composites without impairing the final extent of polymerization.     

Influence of composition on rate of polymerization contraction of light-curing resin composites

A slow contraction may result in reduced gap formation when a restorative resin polymerizes in a dental cavity. It was the aim in the present work to investigate the rate of contraction in relation to composition of experimental light-curing resin composites. The monomer of the resin composites consisted of mixtures of BisGMA, TEGDMA, and in one series HEMA. The resins contained varying amounts of initiators, co-initiators, and inhibitor, and were made composite by adding a silanized glass filler to a content of 74% by weight of the composite paste. The polymerization contraction up to 120 sec was determined by means of the bonded-disk method. Within the ranges studied, the concentration of initiator and co-initiator in the monomer mixture had only an insignificant influence on rate of polymerization. In comparison to camphorquinone, the initiators 1-phenyl-1,2-propanedione and benzil reduced the rate of polymerization without affecting the final contraction. In comparison to N,N-dimethyl-p-aminobenzoic acid ethyl ester, N,N-cyanoethyl methylaniline was as effective, while N,N-diethanol-p-toluidine was less effective as co-initiator. A relatively high content of the inhibitor methoxyhydroquinone reduced the initial rate but not the final polymerization contraction. The rate of polymerization increased with the level of HEMA and TEGDMA in the monomer mixture. It was concluded that intrinsic slow cure may be obtained with certain compositions of resin composites without impairing the final extent of polymerization.     

Quantum yield of conversion of the photoinitiator camphorquinone.

The primary absorber in dental resins is the photoinitiator, which starts the photo polymerization process. We studied the quantum yield of conversion of camphorquinone (CQ), a blue light photoinitiator, in dental resin composites using a LED lamp (3M FreeLight) and a Quartz Tungsten Halogen (QTH) lamp (VIP) as the light curing units at five different irradiances. The molar extinction coefficient, epsilon(469), of CQ was 46+/-2 cm(-1)/(mol/L) at 469 nm. The reciprocity of irradiance and exposure time holds for changes of CQ absorption coefficient, that is, irradiance x exposure time (=radiant exposure)=constant. Both LED and QTH lamps yielded the same curing threshold (the radiant exposure when CQ absorption drops to 1/e) and the same quantum yield conversion under different irradiances. In our dental resin formulation (0.7 wt.% CQ with reducing agents 0.35 wt.% dimethylaminoethyl methacrylate (DMAEMA) and 0.05 wt.% butylated hydroxytoluene (BHT)) the quantum yield was measured as 0.07+/-0.01 CQ conversion per absorbed photon.     

Photopolymerization of N,N-dimethylaminobenzyl alcohol as amine co-initiator for light-cured dental resins.

OBJECTIVE: The present study was carried out in order to assess the suitability of N,N-dimethylaminobenzyl alcohol (DMOH) as co-initiator of camphorquinone (CQ) and 1-phenyl-1,2-propanedione (PPD) in light-cured dental resins. METHODS: DMOH was synthesized and used as co-initiator for the photopolymerization of a model resin based on {2,2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl]propane} (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA). Experimental formulations containing CQ or PPD in combination with DMOH at different concentrations were studied. The photopolymerization was carried out by means of a commercial light-emitting diode (LED) curing unit. The evolution of double bonds consumption versus irradiation time was followed by near-infrared spectroscopy (NIR). The photon absorption efficiency (PAE) of the photopolymerization process was calculated from the spectral distribution of the LED unit and the molar absorption coefficient distributions of PPD and CQ. RESULTS: DMOH is an efficient photoreducer of CQ and PPD resulting in higher polymerization rate and higher double bond conversion compared with dimethylaminoethylmethacrylate. The PAE for PPD was higher than that for CQ. However, the polymerization initiated by PPD progressed at a lower rate and exhibited lower values of final conversion compared with the resins containing CQ. This observation indicates that the lower polymerization rate of the PPD/amine system should be explained in terms of the mechanism of generating primary radicals by PPD, which is less efficient compared with CQ. SIGNIFICANCE: The DMOH/benzoyl peroxide redox system, has recently been proposed as a more biocompatible accelerator for the polymerization of bone cements based on poly(methyl methacrylate), because cytotoxity tests have demonstrated that DMOH possesses better biocompatibility properties compared with traditional tertiary amines. The results obtained in the present study reveal the suitability of the CQ/DMOH initiator system for the polymerization of light-cured dental composites.     

The effect of adding a new monomer “Phene” on the polymerization shrinkage reduction of a dental resin composite

Objective

A new photocurable monomer, “Phene” (N-methyl-bis(ethyl-carbamate-isoproply-α-methylstyryl)amine) was synthesized and incorporated into Bis-GMA/TEGDMA with the aim of reducing polymerization shrinkage swithout detriment to the physical properties and wearing of the resin composites.

Effects of monomer ratios and highly radiopaque fillers on degree of conversion and shrinkage-strain of dental resin composites

ObjectivesThe degree of conversion (DC) and polymerization shrinkage of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as an increase in monomer conversion leads to a high polymerization shrinkage. This paper aims to determine the effect of opaque mineral fillers and monomer ratios on the DC and the shrinkage-strain of experimental composites based on (BisGMA/TEGDMA) monomers (traditionally used monomers). A relationship between the shrinkage-strain and the degree of conversion values was also investigated. The radiopacity of these experimental composites has been investigated in a previous paper.MethodsExperimental resin composites were prepared by mixing different monomer ratios of (BisGMA/TEGDMA) with Camphoroquinone and dimethyl aminoethyl methacrylate (DMAEMA) as photo-initiator system. Five different radiopacifying filler agents: La₂O₃, BaO, BaSO₄, SrO and ZrO₂ at various volume fractions ranging from 0 to 80 wt.% were added to the mixture. The degree of conversion of experimental composites containing different opaque fillers contents was measured using FTIR/ATR spectroscopy. The shrinkage-strain of specimens, photopolymerized at circa 500 mW/cm², was measured using the bonded-disk technique at room temperature with respect to time.ResultsThe result revealed that the DC and the shrinkage-strain decrease slightly with the increasing of opaque fillers loadings, but this decrease is not significant. However, these two properties are closely related to the monomer concentration of the organic matrix. The results have also showed a linear correlation between the shrinkage-strain and DC of experimental composites investigated.SignificanceThe nature and the volume effects of the opaque fillers on the DC and shrinkage of the experimental composites investigated were not significant. However, this study has confirmed the importance of viscosity in the system and shrinkage behavior of dimethacrylate monomers studied. Then we confirmed that direct relationships linked the shrinkage and the DC of filled dental resin composites.     

Effect of different photoinitiator systems on conversion profiles of a model unfilled light-cured resin.

OBJECTIVE: The aim of this study was to compare the efficiency of different camphorquinone (CQ)/amine photo-initiating systems for the photopolymerization of a model dental resin based in Bis-GMA/TEGDMA. METHODS: The monomer conversion versus exposure time was measured in resins containing different types and concentrations of photoinitiators and subjected to different irradiation procedures. The conversion profiles during photopolymerization were investigated using near-infrared spectroscopy (NIR). CQ was used in combination with dimethylaminoethylmethacrylate (DMAEMA), ethyl-4-dimethylaminobenzoate (EDMAB), 4-(N,N-dimethylamino)phenethyl alcohol (DMPOH), and N,N-3,5-tetramethylaniline (TMA) at different concentrations. RESULTS: From the conversion profiles, the approximate order of the accelerating ability of the respective amines were EDMAB approximately DMPOH>TMA>DMAEMA. The conversion profiles of all resins were sensitive to the illumination period. For a given exposure time, samples cured by sequential illumination resulted in lower conversion than samples cured by continuous irradiation. These results were attributed to a combination of both photo and thermal effects. NIR demonstrated to be a convenient method to follow the evolution of the monomer conversion during the photopolymerization of dental resins and, consequently, it is of great utility as a method for determining the relative efficacy of different photoinitiator. SIGNIFICANCE: The DMPOH/benzoyl peroxide redox system, has been recently proposed as a more biocompatible accelerator for the polymerization of acrylic resins. The results obtained in the present study reveal the potential usefulness of the DMPOH amine in light-cured dental composites.     

Organosilicon dental composite restoratives based on 1,3-bis[(p-acryloxymethyl) phenethyl] tetramethyldisiloxane.

OBJECTIVES: The major concern associated with the use of polysiloxanes as polymer matrices in dental restorative materials, is the generally modest mechanical properties of the polymers. However, it has long been demonstrated that thermal stability, and mechanical properties of polysiloxanes can be substantially modified by incorporation of bulkier substituents such as phenyl groups or more polar groups in the chains. The purpose of this research was to evaluate visible light activated dental composites based on the high molecular weight siloxane monomer 1,3-bis[(p-acryloxymethyl) phenethyl] tetramethyldisiloxane (BAPD). METHODS: Hardness, diametral tensile strength (DTS), degree of conversion (DC), water sorption (WS) and polymerization shrinkage of BAPD-based composites and bis-GMA-based composites were determined and compared. RESULTS: Composites based on BAPD exhibited low WS, high DC, low polymerization shrinkage, and had hardness and DTS values that were not significantly lower than those of dental composites based on bis-GMA. SIGNIFICANCE: BAPD is a high molecular weight monomer (MW = 511) with a low viscosity. It did not require the use of low molecular weight diluent monomers in formulating composite resins. The DC of BAPD was high, ranging from 86 to 94%. Although the DC of BAPD was significantly higher than the conventional difunctional dental monomers, the polymerization shrinkage of the siloxane composites (1.70 - 1.81 vol%) was comparable to several composites based on bis-GMA.     

Degree of conversion and microhardness of bulk-fill dental composites polymerized by LED and QTH light curing units

ObjectivesThe degree of monomer conversion is crucial in determining the mechanical and clinical performance of dental resin composites. This study investigated the polymerization adequacy of two bulk-fill resin composites polymerized by Quartz-Tungsten-Halogen (QTH) and Light Emitting Diode (LED) light curing units at different depths.MethodsTwo bulk-fill resin composites (X-tra Fil; Voco and Tetric N-Ceram Bulk-fill; Ivoclar-Vivadent) with diameters of 7 mm and thicknesses of 1–4 mm were prepared and light-cured by LED or QTH. Then, the degree of conversion (DC) and microhardness of the two bulk-fill composites were evaluated.ResultsThe microhardness of X-tra fill was significantly higher than that of Tetric N-Ceram polymerized by LED or QTH. The microhardness and DC of X-tra fil exhibited no significant difference among the increments regardless of type of light source. The DC, however, significantly decreased in deep increments for Tetric N-Ceram polymerized by QTH.ConclusionsThe polymerization efficacies of the two bulk-fill composites were different in terms of the depth of cure and type of light source. The DC and microhardness of the X-tra fill bulk-fill composite polymerized by either QTH or LED did not decrease up to a thickness of 4 mm. Thus, new generations of LED light sources are better options for polymerizing the bulk-fill resin composites than QTH.     

Influence of different concentrations of an iodonium salt on properties of amine-free resins

Objectivesto evaluate the effect of different concentrations of an iodonium salt on the degree of conversion, polymerisation rate, flexural strength and modulus of an amine-free photopolymerizable resin system.MethodsTwo series of UDMA/TEGDMA resin mixtures (1:1 mass ratio) containing 0, 0.5, 1.0 or 2.0 mol% of bis(4-methyl phenyl)iodonium hexafluorophosphate(BPI): and 1 mol% camphorquinone (CQ) were evaluated. One of the series contained 2 mol% of the amine (2-(dimethylamino)ethyl methacrylate-DMAEMA), while the other series had only CQ and BPI as photoinitiators. Polymerisation kinetics and degree of conversion (DC) were evaluated by Fourier transform near-infrared spectroscopy (n = 3). Flexural strength (FS) and modulus (E) were evaluated by a three-point bending method (n = 12). Data were analysed by two-way ANOVA and Tukey’s test.ResultsThe system containing CQ + DMAEMA + BPI provided the highest DC and FS. Amine-free resins had similar E regardless of the presence of BPI. The resins containing CQ+DMAEMA with 1 and 2 mol% of BPI presented higher E compared with others.SignificanceAmine-containing resins reached statistically higher DC, rate of polymerisation and FS than amine-free systems. The use of higher BPI concentrations did not improve the properties of amine-free resins.     

Influence of photoinitiator type on the rate of polymerization, degree of conversion, hardness and yellowing of dental resin composites

OBJECTIVES: To evaluate the degree of conversion (DC), maximum rate of polymerization (Rpmax), Knoop hardness (KHN) and yellowing (b-value) of resin composites formulated with phenylpropanedione (PPD), camphorquinone (CQ), or CQ/PPD at different concentrations. The hypotheses tested were (i) PPD or CQ/PPD would produce less Rpmax and yellowing than CQ alone without affecting DC and KHN, and (ii) Rpmax, DC, and KHN would be directly related to the absorbed power density (PDabs). METHODS: CQ/amine, PPD/amine and CQ/PPD/amine were used at low, intermediate and high concentrations in experimental composites. Photoinitiator absorption and halogen-light emission were measured using a spectrophotometer, Rp with differential scanning calorimetry (DSC), DC with DSC and FTIR, KHN with Knoop indentation; and color with a chromameter. The results were analyzed with two-way analysis of variance (ANOVA)/Student-Newman-Keul's test (p<0.05). Correlation tests were carried out between PDabs and each of DC, Rpmax and KHN. RESULTS: The PDabs increased with photoinitiator concentration and PPD samples had the lowest values. In general, maximum DC was comparable at intermediate concentration, while Rpmax and KHN required higher concentrations. DC was similar for all photoinitiators, but Rpmax was lower with PPD and CQ/PPD. PPD produced the lowest KHN. Yellowing increased with photoinitiator concentration. PPD did not reduce yellowing at intermediate and/or high concentrations, compared to CQ-formulations. PDabs showed significant correlations with DC, Rpmax and KHN. CONCLUSION: PPD or CQ/PPD reduced Rpmax in experimental composites without affecting the DC. The use of PPD did not reduce yellowing, but reduced KHN. DC, Rpmax and KHN were dependent on PDabs.     

Properties of silorane-based dental resins and composites containing a stress-reducing monomer.

OBJECTIVE: To evaluate properties of silorane-based resins and composites containing a stress reducing monomer. METHODS: Resin mixtures and composites were formulated containing (a) a developmental stress reducing monomer [TOSU; Midwest Research Institute]; (b) Sil-Mix (3M-ESPE); (c) photo cationic initiator system. Standard BISGMA/TEGDMA resin (B/T) and composite (Filtek Z250) were used as controls. Polymerization volume change was measured using a NIST mercury dilatometer and polymerization stress using an Enduratec mechanical testing machine. Three point bend tests determined flexural elastic modulus, work of fracture, and ultimate strength (ADA 27; ISO 4049). Fracture toughness was measured using ASTM E399-90. Four groups of resins and composites were tested: Sil-Mix, methacrylate standard, and Sil-Mix with two addition levels of TOSU. An ANOVA was used and significant differences ranked using Student-Newman-Keuls test (alpha=0.05). RESULTS: Polymerization stress values for resins containing TOSU were significantly less than the other materials. Polymerization shrinkage values for Sil-Mix formulations were significantly less than for B/T, but were not different from each other. TOSU-containing formulations generally had somewhat lower mechanical properties values than Sil-Mix or B/T. Polymerization stress values for Sil-Mix-based composites were significantly less as compared to Z250. The 1wt.% TOSU composite had the lowest stress. No difference between composite groups was noted for fracture toughness or work of fracture. For ultimate strength, the 5wt.% TOSU formulation differed significantly from Z250. All Sil-Mix formulations had elastic modulus values significantly different from Z250. SIGNIFICANCE: The ability of TOSU to reduce polymerization stress without a proportional reduction in mechanical properties provides a basis for improvement of silorane-based composites.     

Studies on MMA-tBB resin. I. Comparison of TBB and other initiators in the polymerization of PMMA/MMA resin

Polymerization characteristics of poly (methyl methacrylate)(PMMA)/(methyl methacrylate) (MMA) resin initiated by tributylborane (TBB) were compared with those by benzoyl peroxide (BPO)/N,N-dimethyl-p-toluidine and camphorquinone (CQ)/N,N-dimethylaminoethyl methacrylate from the aspects of temporal changes of residual MMA and molecular weight up to 4 weeks at 37 degrees C. Residual MMA 30 min after polymerization decreased from 8.15% for TBB resin, 8.39% for the BPO resin, and 9.19% for the CQ resin to 0.48%, 3.54%, and 6.79%, respectively, after 4 weeks. The molecular weights at 30 min and 4 weeks after polymerization were 409 x 10(3) and 247 x 10(3) for TBB resin, 297 x 10(3) and 282 x 10(3) for the BPO resin, and 267 x 10(3) and 231 x 10(3) for the CQ resin, respectively. The present results revealed that the TBB-initiated polymerization had unique and different characteristics compared with those initiated by the other common initiators: (1) The decrease in residual MMA was fast, sustained for a long time and resulted in very low value; (2) high molecular weight PMMA was formed first and then decreased with time.     

Evaluation of dental restorative composites containing polyhedral oligomeric silsesquioxane methacrylate.

OBJECTIVES: The aim of this study was to explore novel polymeric dental restorative composites, in which polyhedral oligomeric silsesquioxane methacrylate monomer (POSS-MA) was used to partially (or completely) replace the commonly used base monomer 2,2'-bis-[4-(methacryloxypropoxy)-phenyl]-propane (Bis-GMA). METHODS: The composites were cured (hardened) by photo-initiated free radical polymerization. Mechanical properties (i.e. flexural strength, Young's modulus and diametral tensile strength) of the composites were tested by a universal mechanical testing machine; photopolymerization induced volumetric shrinkage was measured using a mercury dilatometer; and near infrared (NIR) technique was used to study the degree of methacrylate double bond conversion and photopolymerization rate. RESULTS: Small percentage POSS-MA substitution of Bis-GMA (i.e. mass fraction of 10% or less) in the resin system improved the mechanical properties of the composites; while large amount substitution led to less desirable mechanical properties, lower methacrylate double bond conversion, and slower photopolymerization rate. Statistical examinations showed the maximum flexural strength of the composites occurred when 10% (mass fraction) of Bis-GMA was replaced by POSS-MA, while the highest modulus occurred when the mass fraction of POSS-MA was 2%. SIGNIFICANCE: Polymeric dental restorative composites with improved mechanical properties may be designed by judicious choice of monomer (POSS-MA, Bis-GMA and TEGDMA) compositions.