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Rodrigo F. We prepared 25 specimens 2x4 mm with a stainless steel mold. The specimens were fabricated by a metal matrix, filled with resin in two increments and each increment polymerized according to their group. For polymerization of the last layer, we used a glass plate on a polyester strip.
Then the specimens were evaluated for surface microhardness Knoop. Hypothesis was partially confirmed. Se prepararon 25 muestras 2x4mm de un molde de acero inoxidable. These materials present an improvement of mechanical and esthetic characteristics that promote an increase of possibilities to indicate these materials.
The indications for using composites have become greater due to, principally, improvement of its physic-mechanical, chemical and esthetic features Reis et al. It may be used anterior and posterior dental restorations Beazoglou et al. Rupture and wear of these composites are done by chewing forces and infiltration which is caused by stress of shrinkage in the polymerization.
These events above are the main negative points which compromise the composites when used for replacing other materials in posterior dental restorations, especially in extensive cavities Santana et al. Several features of resin composites for direct use are inadequate when used for large restorations in posterior teeth, for example, higher shrinkage of polymerization.
Indirect restorations showed lower effects regarding shrinkages related to direct restorations. These indirect ones are made on the plaster die, silicon or polyether that decrease the shrinkage which is ranging for 2 to 4. In addition, this shrinkage occurs outside of dental cavity Ruyter, Secondary polymerization process after polymerization through heat, or heat and light treatment which these type of restorations have undergone lead to greater stability to this system.
These composites give esthetic with large spectrum of colors, chroma and opacity, biocompatibility and tissue preservation. A possible way to minimize problems related to stress, infiltration and polymerization is to use indirect laboratory composite resin which is called Ceromers.
It represents a large evolution for clinical practice being another alternative instead o using traditional indirect restorations materials such as metals, ceramics, metaloplastics and metaloceramics. On indirect technique, lost dental structure is reconstructed on a cast and the material is cured under controlled conditions in the laboratory. It allows the use of higher irradiances on light curing process that allow the light to penetrate deeper in the material used.
These methods improve mechanical properties as hardness and flexion resistance due to an increase in the degree of conversion of monomers. In addition, it allows better occlusal and proximal accuracy facilitating an insert in the posterior cavity Lovell et al. Although, direct and indirect composites have similar chemical composition, the indirect ones present better mechanical properties Kakaboura et al. Usually, this additional heat treatment for indirect composites is done on special ovens associated to light and heat increasing the cost of indirect restorations.
An alternative for this issue could be the use of direct composites to make the indirect ones. As a result, indirect teeth preparation as inlay, onlay and overlay are done with lower cost Moraes et al. Recent studies Arossi et al.
Consequently, some types of these composites for direct use have been more indicated for indirect restorations.
In the preparation of direct restorations with composites, polymerization is done by light cure, mean wave length is nanometer. This type of curing the composites presents an advantage of being fast, safe and compatible cost comparing to restoration procedure. Several studies demonstrated that additional polymerization improves the mechanical characteristics of composites resulting in greater conversion of monomers in polymers.
This additional procedure may be done by specific equipments. Another benefit of this treatment is a contribution for relieving of tension in the polymerization and polishing procedures Sideridou et al. Ideally, temperatures for this treatment should be higher than the temperature for vitrea transition Tg. It allows a significant increase in mobility of polymeric chain which helps additional ligation and tension relief.
However, a overheating may causes degradation of material and color changing of composite Viljanen et al. There is a hypothesis that additional heat treatment results in better mechanical properties of composites due to linking to monomers which did not react to light curing.
The aim of this study is to evaluate an influence of different heating treatments in the superficial strength of two microhybrid resin-based composites. The characteristics and composition of each material are described in table I. Specimen preparation.
After this period, cover slip and polyester strip were removed and samples were light activated according to the protocol for each group Table II. According to each type of resin-based composite direct or indirect use and type of additional heat treatment were selected 5 groups, described in Table II.
Five consecutive and equidistant readings 50 m m were obtained from surfaces of each sample. After obtaining 5 readings for each specimen, an average was done for each sample. Statiscally analysis. Table III presents an average and standard deviation of the composites tested. Table I. Name, type, main composition, volume of inorganic matrix and manufacturer of composites for direct or indirect using studied in this experiment. Table II. Studied groups and light activation protocols, respectively.
Table III. Mean and standard deviation of hardness Knoop of experimental groups according to complementary polymerization method. The characteristics of resin-based composites, such as hardness and strength, are important mechanical properties that promote a satisfactory clinical result to restoration material. These properties are determined for size, volume, particles distribution of load on the matrix and type of immersion Ferracane. The hardness of a material is a relative measurement of its strength to penetration when a constant specific load is applied.
By definition, hardness is a capacity of a material to resist to penetration done by hard tip being direct proportional to mechanical strength and resistance to wear of a material Yap et al. Changes in hardness influence on the state of reaction of setting time for each material and level of polymerization of material Yap et al. The indirect composites usually present higher level of polymerization compared to direct composites.
It happens due to composites are polymerizated in special units enabling polymerization all restoration surfaces. The degree of polymerization of composites affects the hardness of resin matrix. If the degree of conversion of carbon double ligation is greater, higher values of hardness are reached Asmussem, Complete polymerization of composites is determined by the degree of conversion of monomers in polymers indicating the number of metacrilate groups which react with another one in the conversion process.
The factors which influence on the degree of conversion of composite are: curing time, color composite, temperature, composite thickness, type of load, distance between light and composite, quality of light, polymerization shrinkage Albers, The temperature on the polymerization process affects conversion and properties of polymers Cook et al. Higher temperatures increase the radical and mobility o monomers resulting in greater global conversion and in better characteristics of restorations Lovell et al.
It has been demonstrated that temperature causes significant effect on final values of conversion of composites. The results of this study showed that for composites of direct use, the methods of additional activation by stove or microwave showed no significant increase in micro hardness.
This fact may be justified due to complementary activation might increase a conversion degree without any significant influence on increasing physic and mechanical characteristics of composites Asmussem. Relating to additional polymerization by autoclave promotes a significant increase of micro hardness; it may explicated by a high temperature which a material was submitted.
This increase in the micro hardness of direct composites that was done by different methods of complementary activation was also noted in other studies Brosh et al. When methods of complementary polymerization are done, the properties of materials may be improved Soares et al.
An increase of hardness is explained by increasing the conversion degree of composites submitted to temperatures which are similar to the temperature of vitrea transition. It leads to higher mobility of free monomers and greater flexibility of polymeric chain which possibilities new reaction between active radical. Observing the data of this current study, it is possible to conclude that complementary polymerization with autoclave increased a superficial hardness for direct use composites showing results statistically similar to indirect use composites.
Albers, H. Resin Polymerization. In: Albers, H. Tooth-colored restoratives. Principles and techniques. London, BC Decker, Arossi, G. Asmussem, E. Restorative resins: hardness and strength vs.
Bagis, Y. The effect of post-cure heating on residual, unreacted monomer in a commercial resin composite. Beazoglou, T. Economic Impact of. Regulating the Use of Amalgam Restorations. Public Health Rep. Borba, M. Flexural strength and hardness of direct and indirect composites.
Oral Res. Brosh, T. The influence of surface loading and irradiation time during curing on mechanical properties of a composite. Caughman, W. Shedding new light on composite polymerization. Cook, W.
Knoop hardness of enamel and shear bond strength of brackets bonded with composite resin with and without fluoride. The Knoop hardness measurements were performed before and after bonding metal brackets. After bonding, the specimens were submitted to demineralization and remineralization cycling for 14 days. Keywords: Composite resins. Orthodontic brackets. Shear strength. Palavras-chave: Braquetes.
Evaluation of Knoop microhardness of nanoparticle resins according to the light curing apparatus. Since the introduction of composite resins, depth of polymerization has been a concern among researchers since it can adversely affect the materials' physical and mechanical properties if insufficient. This study assessed the degree of Knoop microhardness of three nanoparticle resins polymerized with four types of sources of activation. Sixty test specimens were prepared using standard curing time of 20 seconds as per manufacturers' recommendations. Those specimens were divided into four groups and further divided into subgroups according to type and type of cured resin, respectively.