Objectives: To measure the level of hydrogen peroxide penetration into the pulp chamber and to evaluate its whitening efficacy when the tooth is treated with an innovative double layer of potassium nitrate desensitizing agent and hydrogen peroxide whitening gel. Methods: Extracted human molar teeth (n=160) were collected. Roots were trimmed 3 mm apical to the cemento-enamel junction, the pulp was removed and a cavity prepared, leaving a standardized wall of 2 mm thickness, which retained 100 µL of acetate buffer solution. A standardized whitening area of 6 mm in diameter was established buccally by painting the rest of the tooth with gray nail varnish. The teeth were randomly assigned into four groups. Group 1: no treatment (glycerin gel, negative control); Group 2: double layer of 20 L 5% potassium nitrate (Relief ACP, Philips Oral Healthcare) and 100 L 25% hydrogen peroxide (Zoom Chairside Whitening Gel, Philips Oral Healthcare); Group 3: double layer of 40 L 5% potassium nitrate and 100 L 25% hydrogen peroxide; and Group 4: 100 L 25% hydrogen peroxide (positive control). All groups received three 45-minute sessions of in-office whitening with light activation at 3-day intervals. Hydrogen peroxide penetration was assessed spectrophotometrically using leucocrystal violet and horseradish peroxidase. Color measurements were assessed with VITA Easyshade at baseline, 1-day, and 1-month post-whitening. Tooth color change was measured per Commission Internationale de l'Eclairage methodology. One-way ANOVA with Tukey multiple comparisons adjustment was performed to compare group differences in overall tooth color change (Delta E*) and hydrogen peroxide penetration (type I error = 0.05). Results: Hydrogen peroxide penetration levels were not significantly different between Group 2 and Group 4; however, the levels were significantly higher than Group 1 and Group 3 (p< 0.0001). Groups 2, 3 and 4 showed no differences for overall tooth color change but differed significantly from the negative control group (Group 1) at 1-day and 1-month post-whitening. Conclusions: Hydrogen peroxide penetration was affected when pretreated by potassium nitrate desensitizer; however, the double layer technique did not adversely affect the whitening efficacy.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O objetivo principal desse trabalho foi avaliar o efeito do procedimento de soldagem empregado, com foco na energia de soldagem, no tipo de chanfro empregado e na tÃcnica de soldagem utilizada, sobre o nÃvel e a distribuiÃÃo das tensÃes residuais resultantes em junta submetida à soldagem multipasse. Foi avaliado tambÃm o efeito da corrente e da velocidade de soldagem de forma isolada sobre estas tensÃes, bem como a correlaÃÃo desses resultados com a microestrutura e dureza encontrada. As tensÃes residuais foram medidas utilizando a tÃcnica de difraÃÃo de raios-x, atravÃs de um minidifratÃmetro para mediÃÃo em campo. As anÃlises metalogrÃficas foram realizadas na seÃÃo transversal da junta, atravÃs de microscopia Ãtica e microscopia eletrÃnica de varredura. Foram levantados os perfis de microdureza na seÃÃo transversal da junta. Os resultados mostraram que as tensÃes residuais originadas da soldagem multipasse na superfÃcie das amostras analisadas sÃo compressivas no metal de solda e trativas da ZAC. As tensÃes residuais encontradas na raiz das amostras analisadas se mostraram trativas tanto no metal de solda quanto na ZAC. A energia de soldagem apresentou uma relaÃÃo nÃo linear com as tensÃes residuais encontradas, sendo encontrado que o parÃmetro que apresentou maior influÃncia foi a velocidade de soldagem, o que representa um problema, pois geralmente tal parÃmetro à o menos controlado nas soldagens manuais. As amostras chanfradas em meio-v foram as que apresentaram os menores nÃveis de tensÃes residuais, devido a alÃvios de tensÃo gerados por deformaÃÃes plÃsticas geradas durante a soldagem, fato que nÃo ocorre nas amostras chanfradas em X. A tÃcnica da dupla camada mostrou-se uma ferramenta bastante Ãtil à soldagem do aÃo ASTM A516 Gr70, pois alÃm de promover o refinamento e revenimento da ZAC-GG das amostras soldadas, acrescentou tensÃes residuais compressivas ao longo de toda a superfÃcie analisada das amostras. As microestruturas e durezas encontradas nas juntas foram bem similares para todas as amostras, sendo a ferrita acicular encontrada no metal de solda uma das provÃveis causas da caracterÃstica compressiva das tensÃes residuais encontradas / The main aim of this work was to evaluate the employed welding procedure effect on the level and profile of the final residual stresses on a multipass joint, with emphasis in the welding energy, chamfer and used welding technique. It was also evaluated the effect of the welding current and speed on these stresses, as well as it was correlated the results with microstructure and microhardness. The residual stress measurement was accomplished through X-ray diffraction, using a minidiffractometer for measurement in field. Metallographic analysis was accomplished in the transverse section of the welded joint, using optic microscopy and scanning electron microscopy. It were determined the microhardness profiles in the traverse section of the welded joint. The results showed that the multipass welding residual stresses on the surface of the analyzed samples are compressive in the weld metal and tensile in the HAZ. In the weld root, the welding residual stresses are tensile as in the weld metal as in the HAZ. The welding energy showed a non-linear relationship with the founded residual stresses and it was observed that the most influential parameter was the welding speed, what can be a trouble, since this parameter is the less controlled in manual weldings. The semi-v chamfered samples were the ones that present the fewer levels of residual stresses, due to the stress relief created by plastic deformations during the welding, what do not occur in the X chamfered samples. The double layer technique showed that it can be used in the welding of ASTM A516 Gr70 steel, because, besides promote a refinement and a drawing back of the CG-HAZ, it increased compressive residual stress in the whole surface of the analyzed samples. The founded microstructures and microhardness were similar for all samples and it has believed that the acicular ferrite founded in the weld metal was one of the the main causes for the compressive characteristics of the welding residual stresses in the joints