Microwave-cured Acrylic Resin Research Articles

Estimate Zinc-Oxide Particles on PMMA Mechanical Properties

Background: Poly-methylmethacrylate is the most used dental material for the construction of removable prosthodontic appliances. Unfortunately, it has shown to be lacking two important properties which are radio-opacity and mechanical strength. Zinc-oxide (ZnO) powder is widely used in dentistry and is considered a semiconductor material with the radio-opaque property. Purpose: To determine the effect of ZnO filler powder on some mechanical properties (impact and transverse strength) of microwave-cured acrylic denture base. Approach: The ZnO powder was added to the acrylic polymer powder in two percentages of 2% and 4% by weight. The prepared sample of acrylic resin was evaluated with impact and transverse strength (n=10). ANOVA and Tukey test were used at a P-value of (P≤0.05). Results: Significant reduction in the transverse strength was noticed in the acrylic denture base with the incorporation of ZnO powder as a filler material in both concentrations. However, non-significant differences were observed in the impact strength with the incorporation of the ZnO filler agent. Conclusions: The addition of ZnO radio-opacifier powder as a filler agent could affect the properties (impact and transverse strength) of microwave-cured acrylic resin.

The Effect of Different levels of a Network Reinforced System and Curing Methods on Properties of Different Acrylic Resin Denture Base Materials.

This study aimed to compare the effect of the addition of light-cured fibre SES mesh at different levels (near the polished surface, at the middle, and near the tissue surface) within different acrylic resin denture base materials on the transverse strength and the surface hardness. One hundred and twenty samples were prepared from three types of acrylic resin denture base materials (high impact heat cured, cross-linked heat cured, and microwaved cured acrylic resins) to test the transverse strength and surface hardness. The samples were divided into four groups: Group1(samples without fibre reinforcement, Control group, n = 30); Group 2 (samples reinforced using SES mesh network near the tissue surface of the acrylic resin sample, n = 30); Group 3 (samples reinforced using SES mesh network near to the polished surface of the acrylic resin sample, n = 30); Group 4 (samples reinforced using SES mesh reinforced network at the middle of the acrylic resin sample, n = 30). The data were statistically analyzed using one-way ANOVA and Tukey's post hoc test at a 0.05 level of significance (SPSS software, version 19.0). One-way ANOVA showed a significant difference in the mean values of transverse strength between all levels of fibre mesh applications and without fibre mesh reinforcement (P < 0.05). Tukey's post hoc test showed that mesh-reinforced fibre in Group 4 had the highest mean value, while the control group showed the lowest mean value. One-way ANOVA showed a significant difference in the mean surface hardness values between cross-linked heat-cured and microwave-cured acrylic resins (P< 0.05). There was no significant difference in the mean surface hardness values between all levels of fibre mesh applications and without fibre mesh reinforcement for high-impact heat-cured acrylic resin (P ˃ 0.05). SES-reinforced glass fibre mesh at different levels significantly increased the transverse strength for different acrylic resin materials but had less effect on the surface hardness for all types of acrylic resin materials.

Biological and chemophysical features of specially designed microwave-cured acrylic denture resin

Background/aim Assortment of denture-based materials and their various curing methods have a great impact on their physical and biological properties. However, esthetics is the most significant parameter in prosthodontics, which is highly deteriorated by water sorption, solubility, and porosity. Moreover, porosity has a great effect on both bacterial and fungus colonization on its surface. This study aimed to assess and compare some physical and biological properties of both heat-cured and microwave-cured acrylic denture-based resins.Patients and methods This study was divided into two parts, in vitro and in vivo parts. The heat-cured and microwave-cured acrylic resin materials were used. Both were utilized for specimens’ fabrication in different dimensions according to ADA specifications to assess their physical and biological criteria. Stereo light microscope aided in determining porosity, while both water sorption and solubility were calibrated by employing an electronic-balance device, besides a specific formula for every test individually. In addition, complete dentures were fabricated for 12 edentulous patients from both materials (n=6). Candida albicans colonization was assessed by employing both chromogenic candida agar and laboratory-incubator device.Results The microwave-cured acrylic denture resin exhibited the lower significant mean value than the conventional heat-cured one in water sorption, corrected water sorption and porosity tests as P value more than or equal to 0.05. Furthermore, it had a significant lower mean value in C. albicans colonization during all visits as P value less than 0.05.Conclusion The microwave-cured acrylic resin proved to be better as compared with conventional heat-cured denture resin a regarding porosity, water sorption, and solubility, and it had the least affinity to C. albicans colonization.

FRACTURE RESISTANCE OF RELINED MICROWAVE-CURED HIGH IMPACT VERSUS CONVENTIONAL HEAT-CURED ACRYLIC RESIN MANDIBULAR DENTURE BASES: AN IN VITRO STUDY

Aim: This in vitro study aimed at comparing the fracture resistance of microwave-cured high impact and conventional heat-cured acrylic resin mandibular denture bases, whether relined by a resilient liner or not. Materials and methods: Conventional heat-cured (C) and high impact microwave-cured (H) acrylic resins were used in the fabrication of 24 mandibular denture bases divided into 4 groups (n=6). Half of these bases were 4 mm in thickness to be used unrelined (CU and HU), the other half was prepared in reduced thickness (2 mm thick) to be relined (CR and HR) with addition silicon resilient liner (R). The denture bases were loaded in compression at the canine-premolar areas bilaterally until failure, using a universal testing machine. Data on fracture resistance (N), work of fracture (J) and deflection at fracture (mm), were analyzed by Kruskal-Wallis and Dunn’s test (P≤0.05). Results: Among the unrelined (CU and HU) and the relined (CR and HR) bases, there were insignificant differences regarding the fracture resistance and the work of fracture median values. Fractures/cracks were repeatedly observed either unilaterally or bilaterally at the premolar regions of the bases. The CU bases exhibited the statistically lowest significant median deflection value at fracture compared to all other denture bases which were insignificantly different from each other. Conclusions: Relining of mandibular denture bases with resilient liners reduced their fracture resistance, work of fracture and increased their amount of deflection at fracture regardless to the type of the denture base material. Economically, conventional acrylic resin denture base material is still the material of choice for dentures that would require relining with resilient liners.

Effect of relining on the misfit of microwave-cured acrylic resin denture bases

Objective: The aim of this study was to evaluate the effect of microwave-energy polymerization on the accuracy and glass transition temperature of acrylic resin denture bases submitted to reline procedure. As denture wearers make up a large portion of the population and often cannot afford a new complete denture, the reline procedure may be the best alternative when indicated. Thus, it is important to evaluate possible changes that would contraindicate this technique. Materials and methods: Twenty denture bases were fabricated from microwave-cured acrylic resin (Onda-Cryl) and randomly divided into control and test groups (n = 10). In the control group, the bases were submitted to measurement of misfit. Bases in the test group were worn down internally and relined. Misfit was measured by cutting the set stone cast/denture base at three predetermined points (canine, molar and tuberosity), measuring the gap between the stone cast and the acrylic resin base at three points using the ImageTool®software suite. Differential scanning calorimetry was used to determine Tg. Data were submitted to t-test for independent-samples (p<0.05). Results: The greatest misfit of canine sections was found at the midpoint of control group (0.280 mm); in molar sections, no significant difference was found. In maxillary tuberosity sections, the greatest misfit occurred at the midpoint of test group (0.352 mm). All gaps were below the maximal clinically acceptable misfit value. There was no significant difference in Tg values between groups. Conclusion: Microwave-energy processing does not affect the fit or Tg of relined denture bases.

Impact of nano-TiO2 particles on water sorption and solubility in different denture base materials

Background/aim Denture base materials with all their diversities and the curing method have a massive impact on their physical, mechanical, and biological traits. This contemplate is aimed to both assess and relate water sorption and solubility of heat-cured and microwave-cured acrylic resin denture base materials with and without the addition of titanium oxide nanoparticles (TiO2 NPs). Materials and methods A total number of 80 circular specimens were fabricated for the two tested groups (n=40). Group I was divided into two subgroups: group IA and group IB (20 each) fabricated from heat-cured and microwave-cured acrylic resin, respectively. Another group (group II) was divided into two subgroups: group IIA and group IIB (20 each) as in group I with the addition of TiO2 NPs. Then, the degree of water sorption and water solubility was calibrated by using an electronic balance for all specimens and determined by the aid of a specific formula. Results The present results indicated that there is a significant difference between all groups using one-way analysis of variance test as the P value was less than 0.05. Moreover, Tukey’s post-hoc test was performed and revealed that there was a significant difference between all subgroups except (group IIA and group IIB) in which there was an insignificant difference regarding water sorption and water solubility. Conclusion Within the limitation of this consideration, microwave-cured acrylic resin is superior to the heat-cured one regarding both water sorption and solubility. Moreover, the addition of TiO2 NPs revealed the best outcome.

Influence of dye and nylon fibers on microwave-cured acrylic resin properties

ABSTRACT Objective: The aim of this study is to analyze if both color and nylon fibers have an influence on microwave-cured acrylic resin properties. Methods: Rectangular and disk-shaped specimens were prepared using acrylic resins; medium pink with and without nylon fibers and colorless without nylon fibers. To obtain the rectangular specimens, a stainless steel die was used with the following dimensions: 64 X 10 x 3 mm (± 0.5mm). To obtain disk-shaped specimens, a die 50mm (±0.5mm) in diameter and 0.5mm (±0.05mm) thick was employed. They were randomized to form groups: control (colorless acrylic) and experimental (medium pink, with and without nylon fibers), with each group consisting of ten (10) specimens rectangular in shape and five (5) disk-shaped. They were analyzed in six (6) assays (izod impact strength, n=10; Knoop hardness, n=10; glass transition temperature, n=3; water sorption and solubility, n=5; degree of monomer/polymer conversion, n=1; flexural strength and flexural modulus, n=10). All variables were subjected to the analysis of variance test followed by Tukey's multiple comparison test, at a 5% level of significance. Results: The analysis of the monomer/polymer degree of conversion did not reveal any difference between the three groups of resins (medium pink, with and without nylon fibers and colorless resin); it was approximately 88%. The results did not show significant differences between the groups for each variable (p&gt;0.05). Conclusion: The results showed that color and presence of nylon fibers in acrylic resins did not affect the properties analyzed in this study.

Effect of Disinfection on the Bond Strength between Denture Teeth and Microwave-Cured Acrylic Resin Denture Base.

Denture tooth debonding is a common complication for denture wearers; however, the effect of complete denture disinfection on bonding between denture teeth and acrylic resin remains unclear. The aim of this study was to evaluate the effect of disinfection methods on the bond strength between denture teeth and microwave-cured acrylic resin denture base. Three commercial brands of denture teeth (Trilux, Biolux, Vipi Dent Plus) and one microwave-cured acrylic resin denture base were tested. Each brand of denture teeth was divided into seven groups (n = 6; estimated by partial Eta squared). The specimenss of groups H and Cl were immersed in 1% sodium hypochlorite and 4% chlorhexidine digluconate for 7 days, respectively. In group Br, the specimens were subjected to toothbrush simulation under 200 g of force for 20,000 cycles. In groups Br-H and Br-Cl, the specimens were brushed and further disinfected with 1% sodium hypochlorite and 4% chlorhexidine digluconate, respectively. In control groups 1 (Co1) and 2 (Co2), the specimens were stored in distilled water for 50 ± 2 hours and 7 days, respectively. Shear bond strength testing was performed at the resin/tooth interface in a universal testing machine at a 1 mm/min crosshead speed. The failure pattern was quantified and classified into adhesive, cohesive, or mixed. Data were analyzed using two-way ANOVA and Tukey HSD test (α = 0.05). Disinfection with 1% sodium hypochlorite (p = 0.031), brushing (p < 0.0001), and association of brushing with either 1% sodium hypochlorite (p < 0.0001) or 4% chlorhexidine digluconate (p = 0.01) reduced the bond strength between denture teeth and microwave-cured acrylic resin denture base. All commercial brands of denture teeth presented a similar bond strength (p > 0.05). The failure pattern was predominantly adhesive independent of the disinfection method and denture tooth brand. Disinfection with sodium hypochlorite, brushing, and the association of mechanical and chemical methods reduced the bond strength between denture tooth and microwave-cured acrylic resin denture base.

Acrylic resin disinfection by peracetic acid and microwave energy

Objective: The objective of this study was to evaluate the effectiveness of disinfection methods in microwave and immersion in peracetic acid in heat-cured, self-cured and microwave-cured acrylic resin, contaminated with Candida albicans. Methods: Five specimens were prepared for each type of acrylic resin. All were infected with Candida Albicans, incubated at 37°C for 24 hours. The group which underwent microwave energy was irradiated with a power of 840W for 1 minute and the other group underwent disinfection by soaking of 0.2% peracetic acid for 5 minutes. Results: All samples proved to be contaminated after the incubation period. After the different processes of disinfection, both immersion in 0.2% peracetic acid as microwave irradiation were effective in disinfection of the 3 types of acrylic resins contaminated by Candida Albicans. Conclusion: Concluded that soaking in 0,2% peracetic acid for 5 minutes with microwave irradiation power 840W for 1 minute are effective methods for disinfecting heat-cured acrylic resin, self-cured acrylic resin and microwave-cured acrylic resin, contaminated with Candida Albicans.

Effect of green tea extract and mouthwash without alcohol on Candida albicans biofilm on acrylic resin.

To evaluate the effect of aqueous extract of green tea and the oral antiseptic without alcohol, on Candida albicans biofilm formation to heat-curing acrylic resin plates. Candida is associated with oral candidiasis in poorly cleaned dentures. Standardised specimens of heat-cured (Conv; n = 30) or microwave-cured acrylic resin (Mw; n = 30) were obtained and divided into six groups (n = 10): G1 = Conv resin and green tea aqueous extract, G2 = Conv resin and mouthwash, G3 = control of Conv resin, G4 = Mw resin and green tea aqueous extract, G5 = Mw resin and mouthwash and G6 = control of Mw-cured resin. The specimens were contaminated with 10 ml of Sabouraud dextrose broth inoculated with 0.1 ml of standard suspension containing 1 × 10(6) cells/ml of C. albicans and incubated for 24 h at 37°C. After this period, they were immersed in the aqueous extract or in mouthwash for 15 min. The control groups were treated with sterile distilled water. Aliquots of 0.1 ml were plated on Sabouraud dextrose agar and incubated at 37°C for 24 h. The numbers of colony-forming units per test specimen (CFU/TS) were calculated, and the results statistically analysed by two-way anova and Tukey's tests (5%). Statistically significant difference was observed for the aqueous extract groups (G1; 33.65%) and mouthwash (G2; 17.06%), when compared to control (G3; 100%), for Conv resin. For the Mw resin, there was significant difference between mouthwash (G5; 43.16%) and control (G6; 100%). The aqueous extract of green tea and mouthwash led to a reduction in the number of viable fungal cells in biofilm formed on acrylic resin.

Effect of triazine derivative added to denture materials on a microcosm biofilm model

The aim of this study was to assess the effect of triazine incorporation to denture materials on biofilm formation of saliva derived from microcosms of patients who are positive for Candida albicans. Biofilms were formed on microwave-cured acrylic resin, one hard denture liner, and two soft denture liners containing 0, 2.5, 5, and 10% triazine. For experimental subset (n = 10), mechanical properties of the materials and colony-forming unit counts from the biofilms formed on the materials were assessed. Flexural strength and modulus decreased with the addition of 2.5% triazine (p < 0.01). In general, the addition of 5 and 10% triazine leaded to more soluble materials (p < 0.001). Saliva donor with candidiasis resulted in higher counts of total microorganisms (p = 0.0294) and Streptococci (p = 0.0008). Soft denture liners showed the highest counts for total microorganisms, Streptococci, and Candida species (p < 0.001). The addition of triazine directly to denture materials was not beneficial in reducing biofilm formation in a complex biofilm model.

Evaluation and Comparison of Effect of Delmopinol Application on Adherence of Candida albicans on Denture Fitting Surface on Three Types of Acrylic Resin: An in vitro Study

ABSTRACTPurposeDenture-related stomatitis is probably the most common form of oral candidiasis and its reported prevalence varies widely ranging up to 65%. In thisin vitrostudy, we evaluate the effect of delmopinol application on Candida albicans adherence on heat-cured acrylic resin, cold-cured acrylic resin and microwave-cured acrylic resin.Materials and methodsA total of 40 specimens of each type of acrylic resin were made; 20 specimens of each type were contaminated before delmopinol treatment and 20 specimens were contaminated after delmopinol treatment. The each specimen in each tube was individually transferred to a spectrophotometer at 530 nm wavelength in order to measure the turbidity degree, through the transmittance. Aliquots of 10 μl of each tube was then collected and inoculated into agar Sabouraud plates containing 500 mmol/l of sucrose, which was incubated for 24 hours at 37oC, in order to check microbial growth. Two-way ANOVA analysis of variance test and posthoc Turkey's test were carried out to ascertain the level of significance (p &lt; 0.001) of various observations.ResultsMean of the turbidity degree of contamination after delmopinol application for heat-cured acrylic resin specimens (A1) 1440.80, colony forming units (CFU) (⨯106/ml), cold-cured acrylic resin specimens (B1) 833.30 CFU (⨯106/ml) and microwave-cured acrylic resin specimens (C1) 944.70 CFU (⨯106/ml) was significantly higher than the mean of the turbidity degree of contamination before delmopinol application for heatcured acrylic resin specimens (A2) 550.85 CFU (⨯106/ml), coldcured acrylic resin specimens (B2) 341.45 CFU (⨯106/ml) and microwave-cured acrylic resin specimens (C2) 451.50 CFU (⨯106/ml).ConclusionIn case of contamination after delmopinol application, heat-cured acrylic resin showed maximum reduction in adherence of C. albicans in the study.How to cite this articleShakya P, Jain D, Rahangdale T. Evaluation and Comparison of Effect of Delmopinol Application on Adherence of Candida albicans on Denture Fitting Surface on Three Types of Acrylic Resin: An in vitro Study. Int J Prosthodont Restor Dent 2012;2(4):129-135.

In vitro antifungal action of different substances over microwaved-cured acrylic resins

Objective:The presence of Candida albicans on the surfaces of denture-base acrylic resins is strongly related to the development of oral stomatitis. This study evaluated the antifungal action of different agents over microwave-cured acrylic resin without polishing specimens previously contaminated with Candida albicans.Material and Methods:Sixty specimens were immersed in BHI broth previously inoculated with the yeast and stored for 3 h at 37°C. They were divided into 5 experimental groups (n=10): G1: 2% chlorhexidine solution (10 min); G2: 0.5% sodium hypochlorite (10 min); G3: modified sodium hypochlorite (10 min); G4: effervescent agent (5 min); G5: hydrogen peroxide 10v (30 min). The specimens of the control group 1 (C1) were not disinfected. Ten additional specimens of the control group 2 (C2) were not infected with the yeast, aiming to check the asepsis during the experiment. The disinfection agents were neutralized and the acrylic resin specimens were immersed in BHI Broth for 24 h. Culture media turbidity was evaluated spectrophotometrically according to the transmittance degree, i.e. the higher the transmittance the stronger the antimicrobial action. Statistical analysis was performed (Kruskal-Wallis Test, p<0.05).Results:The results, represented by the medians, were: G1 = 40; G2 = 100; G3 = 100; G4 = 90; G5 = 100; C1 = 40; C2 = 100.Conclusions:This in vitro study suggested that sodium hypochlorite-based substances and hydrogen peroxide are more efficient disinfectants against C. albicans than 2% chlorhexidine solution and the effervescent agent.

Study of the surface hardness and modulus of elasticity of conventional and microwave-cured acrylic resins

The aim of this study was to evaluate the following acrylic resins: Clássico, QC-20 and Lucitone, recommended specifically for thermal polymerization, and Acron MC and VIPI-WAVE, made for polymerization by microwave energy. The resins were evaluated regarding their surface nanohardness and modulus of elasticity, while varying the polymerization time recommended by the manufacturer. They were also compared as to the presence of water absorbed by the samples. The technique used was nanoindentation, using the Nano Indenter XP, MTS. According to an intra-group analysis, when using the polymerization time recommended by the manufacturer, a variation of 0.14 to 0.23 GPa for nanohardness and 2.61 to 3.73 GPa for modulus of elasticity was observed for the thermally polymerized resins. The variation for the resins made for polymerization by microwave energy was 0.15 to 0.22 GPa for nanohardness and 2.94 to 3.73 GPa for modulus of elasticity. The conclusion was that the Classico resin presented higher nanohardness and higher modulus of elasticity values when compared to those of the same group, while Acron MC presented the highest values for the same characteristics when compared to those of the same group. The water absorption evaluation showed that all the thermal polymerization resins, except for Lucitone, presented significant nanohardness differences when submitted to dehydration or rehydration, while only Acron MC presented no significant differences when submitted to a double polymerization time. Regarding the modulus of elasticity, it was observed that all the tested materials and products, except for Lucitone, showed a significant increase in modulus of elasticity when submitted to a lack of hydration.

Polymerization time for a microwave-cured acrylic resin with multiple flasks.

This study aimed at establishing the polymerization time of a microwave-cured acrylic resin (AcronTM MC), simultaneously processing 2, 4, and 6 flasks. Required time was measured according to the parameters: monomer release in water, Knoop hardness, and porosity. Samples were made with AcronTM MC in different shapes: rectangular (32 x 10 x 2.5 mm) for monomer release and porosity; and half-disc (30 mm in diameter x 4 mm in height) for Knoop hardness. There were four experimental groups (n = 24 per group): G1) one flask (control); G2) two flasks; G3) four flasks, and G4) six flasks. At first, polymerization protocol was similar for all groups (3 min/450 W). Time was then adjusted for G2, G3, and G4, based on monomer release evaluation in the control group, obtained by spectrophotometer Beckman DU-70, with emitting wave of 206 nm. Knoop hardness test was performed using a Shimadzu HMV 2000 hardness tester, and 10 indentations were performed on each specimen's surface. Porosity was assessed after specimens were immersed in black ink and the pores counted in a microscope. Results showed that the complete polymerization of the resin occurred in 4.5 min for two flasks (G2); 8.5 min for four flasks (G3); and 13 min for six flasks (G4), all with 450 W. Statistical analysis revealed that the number of flasks does not interfere with polymerization, Knoop hardness, and porosity of the resin. Results showed that polymerization of microwave-curing resin with more than one flask is a viable procedure, as long as polymerization time is adjusted.

Influence of number and position of flasks in the monomer release, Knoop hardness and porosity of a microwave-cured acrylic resin.

Simultaneous irradiation of multiple flasks may have deleterious effects on the polymerization of microwave cured-acrylic resins. This study evaluated the effect of the number and position of flasks in the monomer release, Knoop hardness and porosity of a microwave-cured acrylic resin. Samples were made of Acron MC(R) (AMC, GC Dent. Ind. Corp.) processed at 500 W. The following associations of the number and position of the flasks were tested: one simple flask centrally placed on the turning plate (I, control); two flasks, one in the centre (IIa) and the other peripherally placed in the plate (IIb); two flasks centrally, one above (IIIa) and the other below (IIIb). The processing time varied according to the number of flasks: 3 or 4.5 min for one or two flasks, respectively. For monomer release, each specimen (n = 12) was put in an assay glass tube containing 6 mL of deionized water that was changed daily. Monomer levels were obtained by spectrophotometry at 206 nm. For hardness test (n = 10), 12 indentations were made in the surface of each specimen. Immersing the polished specimens in permanent ink and counting the porous in a stereo light microscope verified the porosity. Monomer release was significant before 24 h: GI = 263.1(153.3)a, GIIa = 236.9(180.2)a, GIIb = 441.5(446.2)a, GIIIa = 1216.6(857.9)b. Tukey test showed no statistically significant differences among the groups for porosity and hardness. Monomer release was affected by the position of the flask (P < 0.05).

Effect of cooling procedure on final denture base adaptation.

Well-fitted dentures prevent hyperplasic lesions, provide chewing efficiency and promote patient's comfort. Several factors may affect final adaptation of dentures, as the type of the acrylic resin, the flask cooling procedure and the water uptake. This investigation evaluated the effect of water storage and two different cooling procedures [bench cooling (BC) for 2 h; running water (RW) at 20 degrees C for 45 min] on the final adaptation of denture bases. A heat-cured acrylic resin (CL, Clássico, Clássico Artigos Odontológicos) and two microwave-cured acrylic resins [Acron MC, (AC) GC Dent. Ind. Corp.; Onda Cryl (OC), Clássico Artigos Odontológicos] were used to make the bases. Adaptation was assessed by measuring the weight of an intervening layer of silicone impression material between the base and the master die. Data was submitted to ANOVA and Tukey's test (0.05). The following means were found: (BC) CL=0.72 +/- 0.03 a; AC=0.70 +/- 0.03 b; OC=0.76 +/- 0.04 c//(RW) CL= 1.00 +/- 0.11 a; AC=1.00 +/- 0.12 a; OC=0.95 +/- 0.10 a. Different labels join groups that are not statistically different (P > 0.05). Comparisons are made among groups submitted to the same cooling procedure (BC or RW). The conclusions are: interaction of type of material and cooling procedure had a statistically significant effect on the final adaptation of the denture bases (P < 0.05); water storage was not detected as a source of variance (P > 0.05) on the final adaptation.

Effect of intrinsic pigmentation on the flexural strength of a microwave-cured acrylic resin.

This study investigated the effect of intrinsic pigmentation on the transverse strength of a microwave-cured acrylic resin. Forty transverse strength specimens were fabricated according to ADA specification #12 and divided into 5 groups. No fibers were added to group I (GI, control group); acrylic stain (AS) was added to GII and GIII in concentrations of 0.5 and 1.5% w/w, respectively; acrylic fibers (AF) were added to groups GIV and GV in concentrations of 0.5 and 1.5% w/w, respectively. All specimens were irradiated in a microwave oven with a cycle of 3 min at 360 W, followed by 4 min resting, then 3 min at 810 W. Flasks were bench-cooled for 30 min at room temperature, followed by immersion in cold tap water for 30 min. After storage in distilled water at 37 degrees C for 48 h, all specimens were tested for flexural strength in a testing machine (EMIC-DL 500) at a crosshead speed of 0.5 cm/min. Mean and standard deviation (MPa) for the flexural strength test were: GI = 86.0 +/- 7.9; GII = 86.0 +/- 9.8; GIII = 86.6 +/- 7.7; GIV = 84.9 +/- 5.3; GV = 84.9 +/- 5.2. No statistical differences were detected among the groups (ANOVA, p > 0.05). The addition of the acrylic fibers or the acrylic stain did not affect the transverse strength of the microwave-cured acrylic resin.

Microwave-cured acrylic resins and silicone-gypsum moulding technique.

This study evaluated the residual monomer (RM), Knoop hardness (KHN) and transverse strength (TS) of two microwave-cured acrylic resins (Acron MC(R) (A), GC Dent. Ind. Corp., Tokyo, Japan and Onda Cryl (O), AO Classico Ltda, São Paulo, Brazil) when processed with an all-type III gypsum moulding technique (G) or a silicone-gypsum moulding technique (S). One hundred and forty four specimens were fabricated and equally distributed into four groups (AG, AS, OG and OS). The TS tests were conducted after 48 h of water storage, and KHN values were obtained after 24, 48, 72 h and 30 days. The RM was determined every 24 or 48 h over a period of 288 h. The acrylic resins were prepared and processed according to the manufacturer's directions. Both AS and OS showed the highest means (P < 0.05) for RM (microg cm-2) only after 24 h (AS = 56.84 +/- 27.39/AG = 7.51 +/- 5.75/OS = 3.59 +/- 1.60/OG = 1.02 +/- 0.3), 48 h (AS = 28.99 +/- 9.35/AG = 2.65 +/- 2.17/OS = 2.37 +/- 0.84/OG = 0.68 +/- 0.49) and 72 h (AS = 15.98 +/- 9.01/AG = 1.40 +/- 0.57/OS = 1.87 +/- 0.52/OG = 0.75 +/- 0.44). Both AS and OS showed the highest means (P < 0.02) for KH after 24 h (AS = 18.69 +/- 2.3/AG = 17.79 +/- 0.7/OS = 18.41 +/- 1.0/OG = 16.04 +/- 0.6). After 48 h the mean values for OS and OG differed significantly (P < 0.03) (OS = 18.67 +/- 0.8/OG = 16.75 +/- 0.8). No differences of KHN were found among the groups during the storage periods. The TS values for A and O were not affected by either G or S (P > 0.05). Silicone-gypsum mould technique affected the RM and KHN of the resins in the first 2 and 5 days of analysis, respectively. The type of mould did not affect TS, and the acrylic resins differed from each other for all properties regardless of the type of mould.

Dimensional stability of denture bases following repair with microwave resin

The dimensional stability of a commercially available acrylic resin, designed specifically for polymerization by microwave irradiation, was compared with that of a conventional water-bath-cured resin. Resin bases were processed on duplicate stone casts prepared from a cobalt chromium master die. Twenty bases were polymerized, using acrylic resin modified for rapid heat curing, in a water-bath at 100°C for 22 min. A further 20 bases were polymerized using a microwave curing acrylic resin, in a conventional microwave oven at 500 W power output for 3 min. Ten bases from each group were sectioned in a parasagittal direction and repaired using the microwave curing resin. Following each curing cycle the fit of the posterior border of each base was evaluated via a silicone index formed between the base and the master die. The index was invested in stone and sectioned through the posterior palatal region to allow measurement of its thickness by means of an eyepiece micrometer. One-way analysis of variance and unpaired Student's t tests were employed to compare the differences in distortion at the initial cure and following repair. No significant differences were found in the distortion of the acrylic resin bases produced from the heat-cured or microwave-cured materials. All bases exhibited significant further distortion on repair with the microwave-cured acrylic resin.