Network structure of Bis-GMA- and UDMA-based resin systems

OBJECTIVES: The commonly used dental base monomers 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenylene]propane (Bis-GMA) and 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane (UDMA) require the use of a diluent monomer, such as triethylene glycol dimethacrylate (TEGDMA). The aim of this study was to measure double bond conversion of UDMA/TEGDMA and Bis-GMA/TEGDMA polymeric systems, determine the leachable portion, and analyze network formation by evaluating crosslinking and pendant double bonds. METHODS: UDMA or Bis-GMA was combined with TEGDMA in systematic increments and irradiated to form light cured polymers. Fourier transform infrared spectroscopy in the near-infrared region was used to measure double bond conversion. The leachable sol fraction was analyzed by 1H NMR. Resin composites were formulated. Flexural strength was measured by three-point bending and volumetric shrinkage was determined with a mercury dilatometer. RESULTS: The amount of base monomer greatly influenced double bond conversion, sol fraction, and crosslinking. Increasing base monomer concentration decreased double bond conversion, increased the leachable fraction, and decreased crosslinking and network formation. At mole fractions higher than 0.125, the UDMA polymers had significantly higher conversion than the Bis-GMA polymers. Bis-GMA polymers had higher leachable amounts of unreacted monomer, while UDMA mixtures had more crosslinking than the Bis-GMA mixtures. In regards to the physical properties of resin composites, increasing the base monomer improved flexural strength and decreased volumetric shrinkage. SIGNIFICANCE: This systematic study for the evaluation of conversion, leachability, crosslinking, and network structure along with physical properties, like volumetric shrinkage and flexural strength, are required for the optimization of competing desirable properties for the development of durable materials.     

Effect of airborne particle abrasion treatment of two types of 3D-printing resin materials for permanent restoration materials on flexural strength

ObjectivesThis study aimed to assess the effects of airborne-particle abrasion (APA) on the flexural strength of two types of 3D-printing resins for permanent restoration.MethodsTwo types of 3D printing resins (urethane dimethacrylate oligomer; UDMA, ethoxylated bisphenol-A dimethacrylate; BEMA) constituting different components were printed. The specimen surfaces were subjected to APA using 50 and 110 µm alumina particles under different pressures. The three-point flexural strength was measured for each surface treatment group, and a Weibull analysis was performed. Surface characteristics were analyzed via surface roughness measurements and scanning electron microscopy. Dynamic mechanical analysis and nano-indentation measurements were limited to the control group.ResultsThe three-point flexural strength according to the surface treatment was significantly lower in the UDMA group for large particle sizes and at high pressures; the BEMA group demonstrated low flexural strength for large particle sizes regardless of the pressure. After thermocycling, the flexural strengths of UDMA and BEMA significantly decreased in the group subjected to surface treatment. The Weibull modulus and characteristic strength of UDMA were higher than those of BEMA under different APA and thermocycling conditions. As the abrasion pressure and particle size increased, a porous surface formed, and the surface roughness increased. Compared with BEMA, UDMA featured a lower strain, greater strain recovery, and a negligible increase in modulus according to strain.SignificanceThus, surface roughness increased with the sandblasting particle size and pressure of the 3D-printing resin. Hence, a suitable surface treatment method to improve adhesion can be determined by considering physical property changes.     

Polymer properties on resins composed of UDMA and methacrylates with the carboxyl group.

The properties of dental matrix resins have been improved by synthesis of new monomers. However, except for improvements in water-resistance, monomers with better mechanical properties than Bis-GMA and UDMA could not being synthesized. Changing the point of emphasis, we tried to improve the mechanical properties controlling the matrix resin higher structure using noncovalent bonds. We prepared a matrix resin structured by UDMA, which is a high viscosity base monomer with imino groups, and by a low viscosity acidic monomer with carboxyl groups, which permits noncovalent bonds such as hydrogen bonds or electrostatic interaction with imino groups. The maximal mechanical strength for matrix resins structured by UDMA and an acidic monomer was obtained with a composition of imino groups and carboxyl groups at a ratio of 1:1. This mechanical strength value was higher than those obtained with UDMA resin or with a Bis-GMA/TEGDMA/UDMA resin with typical composition. The improvement in mechanical properties may be due to the complex based on noncovalent bonds, between the imino groups of UDMA and the carboxyl groups of the acidic monomers.     

Aging studies of light cured dimethacrylate-based dental resins and a resin composite in water or ethanol/water.

OBJECTIVE: To study the aging of neat resins, prepared from bis-GMA, UDMA, D(3)MA or a mixture of bis-GMA/UDMA/D(3)MA (65/20/15 w/w/w), in water or 75% (v/v) ethanol/water 37 degrees C. Also the study of aging of Heliomolar RO, which is a radiopaque, microfilled, light cured composite, the resin matrix of which is the copolymer of bis-GMA/UDMA/D(3)MA (65/20/15 w/w/w). METHODS: Samples of neat resins and Heliomolar RO prepared by light curing were immersed in water or 75% (v/v) ethanol/water 37 degrees C, for 1, 7 or 30 days. Then the flexural and tensile strength were determined. The fractured surface of samples after the flexural tests was observed by scanning electron microscopy. RESULTS: Bis-GMA and copolymer resin did not showed any significant change in mechanical properties after immersion in water or 75% (v/v) ethanol/water 37 degrees C, for 30 days. On the contrary UDMA, D(3)MA and the composite Heliomolar RO showed a significant decrease. SIGNIFICANCE: The results obtained showed that the effect of aging in water or ethanol/water solution on mechanical properties of a light cured dimethacrylate resin depends on the chemical structure of resin. In the case of resin composite this effect depends on the filler-matrix bond strength.     

Physicochemical properties of dental resins formulated with amine-free photoinitiation systems

ObjectiveTo assess the mechanical properties of two different dimethacrylate resin blends containing the photosensitizer camphorquinone (CQ) alone or in combination with one or more synergists including an amine and/or an iodonium.MethodsTwo co-monomer resin blends were formulated using Bis-GMA/TEGDMA and UDMA/TEGDMA, both at 1:1 mass ratio. Each resin blend was divided into four groups, comprising the following four photoinitiation systems: (1) CQ + 2-(dimethylamino)ethyl methacrylate (DMAEMA); (2) CQ + DMAEMA + bis(4-methyl phenyl)iodonium hexafluorophosphate (BPI); (3) CQ; and (4) CQ + BPI. Materials were evaluated for polymerisation kinetics, water sorption, solubility, flexural strength and modulus.ResultsBisGMA/TEGDMA with CQ showed minimal and insignificant degree of conversion and was not tested for water sorption/solubility and mechanical properties. The ternary system (i.e., CQ + DMAEMA + BPI), promoted the highest degree of conversion for each monomer blend. The resins containing amine had higher mechanical properties than the amine free. However, the UDMA amine free resins exhibited greater flexural strength and modulus than the corresponding amine free BisGMA resins. BisGMA/TEGDMA containing CQ + DMAEMA or CQ + BPI had significantly higher water sorption and solubility than the other groups.SignificanceResins containing amine presented better properties than the amine-free systems. The addition of iodonium salt (BPI) improved the degree of conversion of the resins, even without an amine co-initiator. The amine-free initiator system (CQ + BPI) was more effective when used with UDMA versus BisGMA based-resins respectively.     

Analysis of eluate extracted from dental filling resin by high performance liquid chromatography (author's transl)]

Since the residual monomer of cured filling resin has been implicated in toxicological effects, a high performance liquid chromatography (HPLC) was used to analyze the eluate of bisphenol A diglycidylmethacrylate (Bis-GMA) and methyl methacrylate (MMA) type resins presently used. The main chemicals of the eluate from resin into three kinds of solution, water, 85% methanol water solution and 100% methanol were the residual monomers which formely were Bis-GMA, triethylenglycol dimethacrylate and latterly MMA. Residual initiators, benzoyl peroxide and hydroquinone were also determined. This investigation has shown that HPLC is rapid, reliable and an acurate method to analyze the residual monomers of cured dental filling resins.     

Physical properties of light-cured opaque resin. (1) Influence of monomer composition

Light-cured opaque resins with excellent physical properties were prepared using five types of monomer liquid and titanium dioxide as the powder. The five opaque resin monomer liquid had the following monomer compositions. Methyl methacrylate (MMA)/di(methacryloxyethyl) trimethylhexamethylene diurethane (UDMA) = 70/30 (M-U), MMA/neopenthylglycol dimethacrylate (NPG)/UDMA = 45/45/10 (M-N-U), UDMA/MMA = 70/30 (U-M), 2,2-bis (4-methacryloxypolyethoxy phenyl) propane (2.6 E)/2,2-bis [4-(3-methacryloxy-2-hydroxy propoxy) phenyl] propane (Bis-GMA)/triethyleneglycol dimethacrylate (3 G) = 60/35/5(2.6-B-3) and 3 G/UDMA = 70/30 (3-U) by weight. The bond strength, photo-curability and handling properties of the opaque resin were improved. Three MMA-based opaque resins showed nearly the same values in Knoop hardness number, diametral tensile strength and shear bond strength. The depth of cure increased with the decrease in MMA content of monomer composition, while the amount of residual monomer decreased. The 2.6-B-3 opaque resin had nearly the same properties in depth of cure and Knoop hardness number as the 3-U opaque resin. However, the 2.6-B-3 and 3-U opaque resins had a diametral tensile strength more than twice as high as that of the U-M opaque resin. The bond strength of three MMA-based opaque resins showed 0MPa after 5,000 thermocycles, while the 2.6-B-3 opaque resin, about 16 MPa, and the 3-U opaque resin, about 25 MPa. Therefore, the bond strength of the opaque resin was influenced by monomer composition. 3G-UDMA opaque resin showed excellent physical properties and may be clinically acceptable to bond fixed prosthodontic composite.     

Analysis of polymerization behavior of dental dimethacrylate monomers by differential scanning calorimetry

The polymerization and copolymerization activity of dental monomers was investigated using the dynamic differential scanning calorimetry (DSC) method. As aliphatic dimethacrylate monomers, ethyleneglycol dimethacrylates with different lengths of ethyleneglycol and urethane dimethacrylates were used. As aromatic dimethacrylates, Bis-GMA (2,2-bis[4-(3-methacryloxy-2-hydroxypropoxy)phenyl]propane), BMPEPP (2,2-bis(4-methacryloxy poly-ethoxyphenyl)propane), and DMB (1,2-dimethacryloyloxy benzene) were used. Ethyleneglycol dimethacrylates showed sharp exothermic peaks, and the maximum temperatures (Tp) at the exothermic peaks of the DSC curves decreased according to the lengths of ethyleneglycols. Among urethane dimethacrylates, UDMA (dimethacryloxyethyl 2,2,4-trimethylhexamethylene diurethane) and DMPHU (di-(2-methacryloxypropyl) hexamethylene diurethane) showed sharp exothermic peaks, but DMEHU (di-(2-methacryloxyethyl) hexamethylene diurethane) showed an endothermic peak as well as exothermic peaks. This suggests that reactions other than polymerization occurred in the case of DMEHU. The polymerization activity of aromatic dimethacrylates was lower than that of ethyleneglycol dimethacrylates and urethane dimethacrylates. The copolymerization activity of TEGDMA (triethyleneglycol dimethacrylate) and DMB was also investigated using the dynamic DSC method. The DSC curves of the copolymerization between DMB and UDMA or BMPEPP showed broad peaks, but the DSC curves of the copolymerization between TEGDMA and UDMA or BMPEPP showed sharp peaks. The copolymerization activity of TEGDMA was better than that of DMB.     

Synthesis, characterization and evaluation of urethane derivatives of Bis-GMA.

OBJECTIVES: The aims of the study were to synthesize derivatives of Bis-GMA having pendant n-alkyl urethane substituents and to characterize and evaluate their physicochemical properties. METHODS: Stoichiometric amounts of Bis-GMA and n-alkyl isocyanates were reacted in dichloromethane with dibutyltin dilaurate as a catalyst. Volumetric shrinkage, water uptake, degree of vinyl conversion, refractive index and viscosity of resulting urethane monomers and those of Bis-GMA were measured. The flexural strengths of their corresponding homopolymers and that of Bis-GMA were also measured. RESULTS: These types of urethane derivatives of Bis-GMA exhibited lower viscosities and were more hydrophobic than Bis-GMA. Generally, the viscosity of these experimental monomers decreased with increasing chain length of the alkyl urethane substituent. Photopolymerization of the new monomers gave high degrees of vinyl conversion compared to Bis-GMA. The experimental monomers also yielded polymers with lower polymerization shrinkages at equivalent degrees of vinyl conversion, than Bis-GMA. The refractive indices of these urethane derivatives were similar to Bis-GMA, but the flexural strengths of their polymers were lower than that of the Bis-GMA homopolymer, decreasing with increasing chain length of the alkyl urethane substituent. SIGNIFICANCE: Because of their excellent overall properties, these new derivatives of Bis-GMA have potential as dental monomers that can improve many properties of resin based dental materials that utilize methacrylate monomer systems.     

Preparation of a fluorinated dental resin system and its anti-adhesive properties against S. mutans

ObjectivesThe purpose of this study was to characterize physicochemical properties and investigate anti-bacterial adhesion effect of dental resins containing fluorinated monomers.MethodFluorinated dimethacrylate FDMA was mixed with commonly used reactive diluent triethylene- glycol dimethacrylate (TEGDMA) and fluorinated diluent 1 H,1 H-heptafluorobutyl methacrylate (FBMA) separately at a mass ratio of 60 wt./40 wt. to prepare fluorinated resin systems. Double bond conversion (DC), flexural strength (FS) and modulus (FM), water sorption (WS) and solubility (SL), contact angle and surface free energy, surface element concentration, and anti-adhesion effect against Streptococcus mutans (S. mutans) were investigated according to standard or referenced methods. 2,2-bis[4-(2-hydroxy-3-methacryloy-loxypropyl)-phenyl]propane (Bis-GMA)/TEGDMA (60/40, wt./wt.) was used as control.ResultsBoth fluorinated resin systems had higher DC than Bis-GMA based resin (p < 0.05); compared with Bis-GMA based resin (FS, FDMA/TEGDMA resin system had higher FS (p < 0.05) and comparable FM (p > 0.05), while FDMA/FBMA resins system had lower FS and FM (p < 0.05). Both fluorinated resin systems had lower WS and SL than Bis-GMA based resin (p < 0.05), and FDMA/TEGDMA resin system had the lowest WS (p < 0.05) in all experimental resin systems. Only FDMA/FBMA resin system showed lower surface free energy than Bis-GMA based resin (p < 0.05). When the surface was smooth, FDMA/FBMA resin system had lower amount of adherent S. mutans than Bis-GMA based resin (p < 0.05), while after the surface became roughness, FDMA/FBMA resin system had comparable amount of adherent S. mutans as Bis-GMA based resin (p > 0.05).SignificanceResin system prepared exclusively with fluorinated methacrylate monomers reduced the S. mutans adhesion due to their increased hydrophobicity and decreased surface energy., while flexural properties of it should be improved.     

Genotoxicity and cytotoxicity of dental materials in human lymphocytes as assessed by the single cell microgel electrophoresis (comet) assay

OBJECTIVES: Resin monomers may be released from restorative dental materials and can diffuse into the tooth pulp or the gingiva, and can reach the saliva and the circulating blood. Whereas the cytotoxic potential of some components has been clearly documented, possible genotoxicity in human target cells demands further investigation. METHODS: The Comet assay was used to quantify DNA single strand breaks, alkali labile and incomplete excision repair sites in lymphocytes of 10 volunteers. The xenobiotics investigated were 2-hydroxyethylmethacrylate (HEMA), triethyleneglycoldimethacrylate (TEGDMA), urethane dimethacrylate (UDMA), and bisphenol A-glycidyl methacrylate (Bis-GMA) with N-methyl-N'-nitro-N-nitrosoguanidine and dimethyl sulfoxide as controls. DNA migration was quantified using the tail moment according to Olive (OTM) and DNA migration was considered to be elevated at OTM levels above 2. Cytotoxicity was monitored using trypan blue. RESULTS: In the negative controls, OTM ranged between 1.0 and 1.2. With HEMA concentrations above 10(-6)M, TEGDMA 10(-3)M, Bis-GMA 10(-4)M, and UDMA above 10(-6)M relevant enhancements of DNA migration (OTM>2) were achieved. At higher concentrations of up to 2.5x10(-2) induced DNA migration was expressed by OTM of 3.3 for HEMA, 4.5 for TEGDMA, 7.4 for Bis-GMA, and 2.8 for UDMA. Relevant cytotoxic effects were also seen but vitality levels were at a critical range of 71% for Bis-GMA and 73% for TEGDMA, only. SIGNIFICANCE: In higher concentration levels, all tested substances induced significant but minor enhancement of DNA migration in the Comet assay as a possible sign for limited genotoxic effects. However, with the highest levels of DNA migration being combined with elevated cytotoxic effects, a low in vivo genotoxic strain appears to be posed by the resin components.     

Long-term release of monomers from modern dental-composite materials

The elution of monomers from composite materials influences the biocompatibility of dental restorations. The purpose of the present study was to investigate the elution of monomers [bisphenol A glycidyl methacrylate (BisGMA), triethylene glycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA), and bisphenol A (BPA)] from two light-cured materials (nanohybrid and ormocer) and from a chemically cured composite material, after different curing times (0, 20, 40 and 80 s) and different storage periods (24 h, 7 d, 28 d, and 1 yr after curing). Each specimen was stored in 1 ml of 75% ethanol. This medium was renewed after 24 h, 7 d, 28 d, and 1 yr. The ethanol samples were analyzed using liquid chromatography tandem mass spectrometry (LC–MS/MS). The amount of monomers released from the nanohybrid and the chemically cured composite was significantly higher than released from the ormocer. The curing time exerted a significant effect on the release of monomers. For the nanohybrid, less monomer was released after increasing the curing time. For the ormocer, 80 s of curing resulted in a higher degree of monomer release. The effect of storage differed between the monomers. Although the elution of TEGDMA was significantly decreased after storage for 28 d and 1 yr, a similar amount of BisGMA was released at each storage time-point analyzed, even after 1 yr. The present study showed that ormocer released a very small amount of monomers compared with the other materials.     

Durability of commercial composite resin. Chronological change in transverse strength and transverse elastic modulus of composite resin immersed in MeOH

The durability of visible light-cured composite resin was examined. Five kinds of commercial redox type composite resin and 7 kinds of visible light-cured composite resin were used and their base monomers were analyzed by HPLC. After the set products were immersed in MeOH, transverse strength and transverse elastic modulus were measured. Furthermore, the MeOH sorption, solubility in MeOH and main soluble component were examined. The main component of the base monomer in all 5 kinds of redox type was Bis-GMA. Seven kinds of visible light-cured composite resin consisted of 4 kinds of Bis-GMA (including BMPEPP) and 3 kinds of UDMA. Both cases of redox type and visible light-cured type of composite resin, when they were immersed in MeOH, transverse strength and transverse elastic modulus decreased. In the case of redox type (Clearfil posterior new bond), the decrease of transverse strength and transverse elastic modulus was small, that is, its base monomer was Bis-GMA, and large quantities of hybrid type filler were mixed. In the case of redox type, transverse strength and transverse elastic modulus showed a tendency to decrease with the increase of MeOH sorption. Solubility of 12 kinds of these composite resins was 0.25-4.78% and its main component in Pyrofil light bond A was BMPEPP, the residual were coincident with the main component of base monomer.     

Characterization of urethane-dimethacrylate derivatives as alternative monomers for the restorative composite matrix

ObjectiveThe aim was accomplished by a comparative analysis of the physicochemical properties of urethane-dimethacrylate (UDMA) monomers and their homopolymers with regard to the properties of basic dimethacrylates used presently in dentistry. The homologous series of UDMA were obtained from four oligoethylene glycols monomethacrylates (HEMA, DEGMMA, TEGMMA and TTEGMMA) and six diisocyanates (HMDI, TMDI, IPDI, CHMDI, TDI and MDI).MethodsPhotopolymerization was light-initiated with the camphorquinone/tertiary amine system. Monomers were tested for viscosity and density. Flexural strength, flexural modulus, hardness, water sorption and polymerization shrinkage of the polymers were studied. The glass transition temperature and the degree of conversion were also discussed.ResultsHEMA/IPDI appeared to be the most promising alternative monomer. The monomer exhibited a lower viscosity and achieved higher degree of conversion, the polymer had lower water sorption as well as higher modulus, glass temperature and hardness than Bis-GMA. The polymer of DEGMMA/CHMDI exhibited lower polymerization shrinkage, lower water sorption and higher hardness, however it exhibited lower modulus when compared to HEMA/TMDI. The remaining monomers obtained from HEMA were solids. Monomers with longer TEGMMA and TTEGMMA units polymerized to rubbery networks with high water sorption. The viscosity of all studied UDMA monomers was too high to be used as reactive diluents.SignificanceThe systematic, comparative analysis of the homologous UDMA monomers and corresponding homopolymers along with their physico-mechanical properties are essential for optimizing the design process of new components desirable in dental formulations. Some of the studied UDMA monomers may be simple and effective alternative dimethacrylate comonomers.     

Synthesis and evaluation of modified urethane dimethacrylate resins with reduced water sorption and solubility

ObjectivesNew aliphatic and aromatic urethane dimethacrylate monomers containing pendant phenyl methoxy or ethyl substituents were synthesized in order to reduce the water sorption and solubility of urethane dimethacrylate systems. Selected properties including flexural strength, flexural modulus, water sorption and solubility, and water contact angle were evaluated. Hoy's solubility parameters were also calculated to rank copolymer hydrophilicity.MethodsFilled (20%) composite resins were formulated with each of the newly synthesized dimethacrylates as well as the commercially available urethane dimethacrylate monomer, UDMA. Flexural strength, flexural modulus, water sorption and solubility of the urethane composites were evaluated after light-cured specimens were immersed in water for seven days. Water contact angles were measured on the surface of each material. Data were analyzed using ANOVA and Ryan–Einot–Gabriel–Welsch multiple range tests (α = 0.05).ResultsA significant reduction of nearly 30% and 40% in water uptake was observed with composite polymers containing pendant ethyl and phenyl methoxy groups, respectively, compared to UDMA (p < 0.05). Urethane copolymers containing pendant ethyl groups also showed a significant reduction in water solubility (p < 0.05). A positive correlation was found between contact angle and water sorption as well as Hoy's δ_h for hydrogen bonding forces.SignificanceThe results of this study indicate that the incorporation of pendant hydrophobic substituents within the monomer backbone may be an effective method in reducing the water sorption and water solubility of urethane based dimethacrylate systems. The use of Hoy's solubility parameters to determine the relative hydrophilicity of a polymer may be limited by its three-dimensional chemical structure.     

Hardness,Flexural Strength,and Flexural Modulus Comparisons of Three Differently Cured Denture Base Systems

Purpose: This study compared the surface hardness, flexural strength, and flexural modulus of a light‐ and heat‐cured urethane dimethacrylate (UDMA) to two conventional polymethyl methacrylate (PMMA) denture base resins. The effect of less‐than‐optimal processing condition on the hardness of internal and external surfaces of UDMA specimens was also investigated. Materials and Methods: The materials tested were Eclipse (light‐ and heat‐cured UDMA), Meliodent (heat‐cured PMMA), and Probase Cold (auto‐cured PMMA). Eclipse specimens were prepared by adapting the material onto the master cast and light curing in the processing unit for 10 minutes. Meliodent and Probase Cold specimens were prepared according to the manufacturers' instructions. Twenty rectangular specimens measuring 65 × 10 × 2.5 mm³ were prepared for each material. They were stored in water at 37°C for 30 days before testing. The surface hardness was measured using Vickers Hardness (VHN) test, and flexural strength and flexural modulus were measured using a 3‐point bending test. Twenty‐five additional Eclipse specimens were similarly prepared and were processed at various times of less than 20 minutes of curing. Vickers Hardness was determined on both the external and internal surfaces of specimens. Data were analyzed using a one‐way ANOVA for comparisons of hardness, flexural strength, and flexural modulus between the three denture base materials and for hardness values of both the internal and external surface of Eclipse specimens with curing times. Post hoc analyses (Scheffé test) determined the difference between the groups. Student t‐test was used for comparison of hardness between the external and internal surfaces of Eclipse specimens. Results: The hardness (VHN) values were 19.4 ± 0.7, 17.0 ± 0.4, and 16.0 ± 0.4; the flexural strengths (MPa) were 103 ± 4, 78 ± 3, and 63 ± 4; and the flexural moduli (MPa) were 2498 ± 143, 1969 ± 55, and 1832 ± 89 for Eclipse, Meliodent, and Probase Cold materials, respectively. A comparison among the three polymers showed there were significant differences in surface hardness, flexural strength, and flexural modulus (p < 0.05). No significant difference in surface hardness (VHN) between the internal (19.1 ± 0.6 to 19.4 ± 0.7) and external surfaces (18.9 ± 0.4 to 19.2 ± 0.6) of irradiated Eclipse specimens was observed at 10‐, 12‐, and 14‐minute polymerization times. Conclusion: The surface hardness, flexural strength, and flexural modulus of light‐ and heat‐cured UDMA (Eclipse) were significantly higher than the values obtained for heat‐only cured (Meliodent) and auto‐cured (Probase Cold) PMMA denture base systems.     

Influence of polymerization initiator for base monomer on microwave curing of composite resin inlays

Microwave polymerization was used to make composite resin inlays and the effect examined of the concentration of polymerization initiator for the base monomer. The monomers used were 2,2-bis [4-(3-methacriloxy-2-hydroxypropoxy) phenyl] propane (Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA). Bis-GMA and TEGDMA were mixed in a ratio of 6:4 by weight and were separated into five groups. To each group was added benzoyl peroxide (BPO) in the ratios of 0.1, 0.3, 0.5, 0.7 and 0.9 wt% as the polymerization initiator. These were used as the base monomers. The results showed that the degree of conversion of the cured sample increased with increasing concentration of BPO from 0.1 to 0.5 wt%, however there was no significant difference at 0.5, 0.7 and 0.9 wt% (P> 0.01). Compression strength, diametral tensile strength and the Knoop hardness showed a similar tendency as the degree of conversion. No significant difference was recorded in the Knoop hardness between the top and the bottom surfaces (P> 0.01), which suggested a uniform polymerization in the cured sample. Thus, microwave polymerization would be an efficacious method for making resin inlays with the addition of BPO to the base monomer (Bis-GMA:TEGDMA, 6:4). The maximum conversion was found at a concentration of 0.5 wt%.     

A comparison between the wear resistance of three typesof denture resin to three different dentifrices

This study compares the abrasive effects of three dentifrices on three different denture base resins, a heat cured acrylic resin, a self cured acrylic resin and a light activated urethane dimethacrylate‐based resin. Specimens of the resins were subjected to a regimen of mechanical brushing using measured amounts of paste on standard toothbrushes driven by a reciprocating device. The apparent wear of each specimen was assessed qualitatively using scanning electron microscopy and quantitatively using profilometry and reflex microscopy. The study showed that cleaning pastes play a significant role in the wear of conventional acrylic resins, the relative abrasivity of the dentifrices being substrate related. However, wear was less evident on the light activated urethane dimethacrylate resin. Brushing with water alone produced no detectable wear on any of the specimens.     

A partially aromatic urethane dimethacrylate as a new substitute for Bis-GMA in restorative composites.

OBJECTIVES: The objective of this study was to investigate the use of a new, partially aromatic urethane dimethacrylate in visible-light cured resin-based composite restoratives. Selected mechanical properties, such as flexural strength and flexural modulus of elasticity, of model monomer mixtures and composites containing the new urethane dimethacrylate were investigated and compared to the properties of materials that are based on Bis-GMA, at present the most frequently used cross-linker in restorative composites. In addition, the polymerization shrinkage and the water sorption of selected composites were determined. METHODS: The flexural strength, flexural modulus of elasticity, and the water sorption were determined according to ISO 4049:2000. Test specimens (rods: 2 mmx2 mmx25 mm; discs: d=15 mm and h=1 mm) of the investigated composites were prepared in stainless steel molds and light-cured (150 mW/cm2, 2x180 s). The flexural strength and flexural modulus of rods were measured after the samples had been stored under dry conditions or in water for 24 h at 37 degrees C as well as after they had been stored in water for 7 days at 37 degrees C. The water sorption was determined with discs. The polymerization shrinkage was calculated from the densities of the uncured composite pastes and cured composites. RESULTS: Visible light cured mixtures of dimethacrylate diluents with the new urethane dimethacrylate and composites based on these mixtures show a reactivity, flexural strength, flexural modulus of elasticity, polymerization shrinkage and water sorption similar to those of materials that are based on Bis-GMA. The composites did not show any strong deterioration of the mechanical properties after water storage.     

Bis(4-methyl phenyl)iodonium as an alternative component to diphenyliodonium in camphorquinone-based ternary initiating systems

ObjectiveTo evaluate the influence of different co-initiators (diphenyliodonium hexafluorophosphate - DPI - and bis(4-methyl phenyl)iodonium hexafluorophosphate – BPI) on chemical and mechanical properties of resins.MethodsNine experimental resins (50% Bis-GMA and 50% TEGDMA, w/w) with 60 wt% filler particles were formulated. The initiating system used was camphorquinone (CQ-1 mol%) and ethyl dimethylaminobenzoate (EDAB-2 mol%). Experimental groups were established according to DPI and BPI quantities (0.25, 0.5, 0.75, and 1 mol%). The control group was a resin containing only CQ-EDAB. Light transmission through the resin during polymerisation was analysed with a UV–vis spectrophotometer. Real-time polymerisation of the systems was evaluated using an FTIR spectrometer. Real-time polymerisation shrinkage strain was evaluated, and the flexural strength and modulus of materials were obtained by 3-point bending. Experimental groups were statistically analysed by Analysis of Variance and Tukey’s test (α = 0.05). Dunnett’s test was applied to compare experimental groups with control.ResultsLight transmission rapidly increased initially for resins containing DPI or BPI. After 30 s cure, the irradiance on the lower surface of resin specimens was similar for all groups. After 10 s of light irradiation, groups containing DPI and BPI had higher conversion than the control. However, conversion after 120 s post-irradiation was similar for all groups. The rate of polymerisation, shrinkage strain, and the maximum strain rate were higher for groups containing DPI/BPI. The use of iodonium salts increased the flexural strength and flexural moduli of resins.SignificanceDPI and BPI increased resin reactivity similarly. Increased rate of polymerization influenced light transmission through the resin in the first seconds of polymerisation and increased resin shrinkage and rate of shrinkage, as well as flexural strength and moduli.