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Curing Modes
Most dental composites cure via radical chain polymerization in which monomers are converted to polymers. Various types of initiation systems and activation methods can be used to generate a free radical that initiates the polymerization process. They have significant effects on the kinetics of polymerization and the polymer structure, thus affecting various properties of the composites.18 According to the initiation systems or cure mechanisms, composites can be divided into chemically initiated/self-cured, light-activated, heat-cured, or dual-cured composites.19

For self-cured/chemical-cured composites, when the powder-liquid or paste-paste materials are mixed together, polymerization is initiated by and oxidation–reduction initiator system at room temperature. Self-cure composites are composed of a catalyst part containing benzoyl peroxide(BPO) and a base part containing tertiary amine. The tertiary amines, N,N-dimethyl-p-toludine (DMPT) and N,N-dihydroxyethyl-p-toluidine, are also usually used in self-cure composites. When the two parts are mixed, the radicals formed by the reaction of the BPO and amine are able to react with the monomers that would become polymerization later. However, most self-cure composites are now used as resin-based luting cements or core materials, rather than direct restorations.19 In contrast, the light-cured method uses ultraviolet (UV) or visible light to promote initiation reaction of resin polymerization. Light-activated composites undergo polymerization by irradiation via a blue-light-curing unit in the wavelength range of 410–500 nm.12 These days, almost all dental restorative composites contain CQ/amine complex initiation, visible light-cured, which is safer compared with UV-curing systems, and one-component systems. The mechanism of the CQ/amine system is different from that of the BPO/amine chemical initiation system. When exposed to visible light, the CQ undergoes excitation first by energy absorption. The CQ molecule in the excited triplet state rapidly interacts via electron transfer with an amine to constitute an excited complex, and then extracts hydrogen from the amine to make up a new compound. The excitation energy is transferred to the amine molecule during the process, the α-amino-alkyl radical is more efficient at initiating polymerization than the relatively inactive CQ-ketyl radical.20 Heat-cured composites are polymerized by extra-oral cure involving heat that might help reduce the quantity of remaining double bonds resulting in improvements in mechanical properties. In addition, there still exists another kind of dual-cured composites with combination of various curing systems to form polymers.19, 20 They are widely used for cementing endodontic posts and fabricating core buildups (Figure