EVOLUTION AND CURRENT TRENDS IN COMPOSITE RESINS

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Another monomer widely used, accompanied or not Bis -GMA , UDMA Is The ( urethane dimethacrylate ) , its advantage is that it has less f viscosity and Greater Flexibility , What Improves resistance of the resin. ( 15) The UDMA -based Composite Resins can polymerize More Than Bis -GMA -based (16 ) , sin embargo , Soderholm et al . (17 ) indicated that the profundity of curing was lower in certain UDMA -based Composite Resins DUE a UNA difference between the refractive index of light between the monomer and filler .

Filler particles

Are those which provide dimensional stability to the resin matrix and improve its properties . The addition of these particles to the matrix reduces the polymerization shrinkage , water sorption and the coefficient of thermal expansion , providing increased tensile strength , compression and abrasion , increasing the modulus of elasticity ( stiffness ) . (18)

The filler particles are most commonly used quartz or glass barium are obtained in different sizes through different manufacturing processes ( spraying, grinding , milling ) . Quartz particles are twice harder and less susceptible to erosion than glass, plus they provide better adhesion with free agents (Silane ) . Are also used silica particles of a size of approximately 0,04mm ( microparticles ) which are obtained by pyrolytic processes (burning ) or precipitation ( colloidal silica ) . ( 2)

The current trend is decreasing particle size , making the distribution is as close as possible , around 0.05 microns. ( 19)

Importantly, the greater the incorporation of filler to the matrix, would be the best resin properties as it produces less curing shrinkage and consequently lower marginal filtration argument in which the rise of the condensate is based resins . (20)

However , as important as the polymerization shrinkage is tension or polymerization shrinkage stress , that is, the relationship between the contraction of the resin , its modulus of elasticity ( stiffness) and the number of tooth surfaces or walls bind (Factor C ) . With this, the resins with high filler incorporation just less contracting , causing a higher shrinkage stress which leads to higher filtration, for being too rigid . (21)

Agent connection or coupling

In the initial development of composite resins , Bowen (22 ) demonstrated that the optimal material properties, dependent on the formation of a strong bond between the inorganic filler and the organic matrix . The union of these two phases is achieved by coating the filler particles with a coupling agent that has characteristics of both filler and matrix. The head of this binding agent is a bifunctional molecule having ( Si- OH ) silane group at one end and methacrylate groups (C = C ) on the other . Since most commercially available composites have silica-based filler , the coupling agent used is more silane . (23)

The silane used is most often γ- methacryl - oxypropyl trimethoxysilane (MPS) (Fig. 3) , this is a bipolar molecule that binds to the filler particles when they are hydrolyzed by hydrogen bonds and turn holds methacrylates groups , which form covalent bonds with the resin during the polymerization process to provide a suitable interface resin / filler particle . (24)

Also, the silane improves the physical and mechanical properties of the composite resin , because it provides for transfer of the phase voltages is easily deformed ( resin matrix ) , to the more rigid phase ( filler particles ) . Furthermore, these coupling agents prevent water penetration into the BisGMA / interface filler particles , promote hydrolytic stability within the resin. ( 25) have experienced other agents such as 4- META , and zirconates several titinatos however none of these agents proved superior to MPS . (26)

Advances in technology silanization care more than anything in a uniform particle filler which provides improved properties to the composite resin coating. To achieve this uniform coating manufacturers use different ways of covering and lining up three times filler particle . ( 27)

Polymerization Initiator - Activator System

The polymerization process of the monomers in the composite resin can be achieved in several ways. In whatever form the action of free radicals needed to initiate the reaction . For these free radicals are generated need an external stimulus. According Yearn , (28) in the self- cured resins stimulus comes from the mixing of two pastes , one of which has a chemical activator ( aromatic tertiary amine as the dihydroxyethyl -p-toluidine ) and the other an initiator ( peroxide benzoyl ) . In the case of photo - cured systems , the energy of visible light provides the stimulus activates an initiator in the resin ( canforoquinonas , or other diquetonas lucerinas ) . It is necessary that the resin is exposed to a light source with appropriate wavelength between 420 and 500 nm in the visible light spectrum . (29 ) However, the clinician must be careful to minimize exposure to light until the material is ready to heal, otherwise you can start a premature polymerization and working time can be reduced considerably. (30 )

Another common form of the resins is polymerized through the application of heat alone or in conjunction with curing . This procedure is quite common in the resins used in the laboratory for fabrication of inlays and onlays . For heat- cured , temperatures of 100 ° C or more , provided the temperature materials which serves as a stimulus to activate the initiator. Then curing the thermal curing improves properties of the resin especially the wear resistance and resistance to the marginal degradation (31). Either mechanism is efficient and produces a high degree of polymerization under appropriate conditions

CLASSIFICATION OF COMPOSITE RESINS

Over the years composite resins are classified in different ways in order to facilitate the identification and subsequent clinical therapeutic use. A still valid classification is proposed by Lutz and Phillilps (32) This classification based on the size and distribution of the filler particles in resins. Conventional or macrorelleno (particles 0.1 to 100m) microfilled (0,04m particles) and hybrid resins (fillers of different sizes). (Fig. 4).

Another classification system was devised by Willems et al., (33) which despite being more complex, provides further information on various parameters such as Young's modulus, the percentage of inorganic filler (by volume), the size of particles, surface roughness and compressive strength. (Table 1)

1. macrorelleno or