# Photopolymerization of Ceramic Resins by Stereolithography Process: A Review
## 2 Ceramic Resins for Stereolithography
### Components of Ceramic Resins  
Raw resins are characterized by a solution of three main components: monomer, oligomer, and photo-initiator. In particular, photo-initiator (PI) can have two different natures: cationic or free-radical [20]. The nature of ceramic powder brings to require specific chemical properties of the different component used. The polarity of monomers/oligomers, PI type, diluent, and dispersant fraction, need correct evaluations. The principal categories of monomeric bases used for SLA resins are acrylates and epoxides. Distinctions between these two classes are being made in relation to chemical reactions in which they are involved. Acrylic monomers are characterized by a double carbon bond linked to a vinyl group; the polymerization starts when a free-radical PI reacts with a double bond. Epoxide monomers react with a cationic PI, generating a chain reaction that brings to crosslinking. Acrylic and epoxy monomers are the bases of two macro-categories of resins, respectively aqueous and non-aqueous. Monomers are used not only to give particular characteristics to the final object, but also because of their diluent behavior [6; 21]. Monomers and oligomers have got respectively a low and intermediate molecular weight and this parameter influence the final viscosity of the slurry. The polarity of the monomeric/oligomeric base has great importance for the correct dispersion of the solid phase [21]. Monomers and oligomers are unable to aggregate spontaneously, the role of PI is to accelerate this chemical reaction absorbing UV-light [6] (Figure 3). There are two different ways to catalyze the reaction, in the function of the nature of PI [22]:  
Free-radical PIs: when exposed to UV-light generates free radicals, which react breaking the double bond carbons (C=C) of a particular molecule of monomer/oligomer;  
Figure 3: Example of a mixture that undergoes cross-linking when exposed to light. unpaired carbons react with other molecules carbons, starting to generate a long-chain molecule. Macroscopically liquid resin polymerizes and evolve to solid state. This kind of reaction is typical for acrylate-methacrylate bases. Two main types of free-radical reactions are known, \(\alpha\)-cleavage and H-abstraction. In \(\alpha\)-cleavage mechanism double bond brakes and brings to formation of free radicals under the UV energy stimulation; instead, in H-abstraction photo-initiator reacts when stimulated by energy source with a second compound (a co-initiator) through hydrogen or electronical abstraction [20; 21]. Cationic PIs: when exposed to UV-light generates Lewis or Bronsted acids through photolysis, which attack intra-molecule bond starting a re-aggregating chain reaction. This kind of phenomena take place with epoxy-based monomer. Lewis's acids are generated when a diazonium salt are stimulated by an ultraviolet source and the anionic phase separate losing a halogen atom; the crosslinking starts when Lewis's acids react with cycloaliphatic epoxides [23]. Bronsted acids are formed from different bases of salts, such as sulfur and phosphorous, compounds with an unshared pair of electrons. Typical onion salts which generate Bronsted acids are Iodonium and Sulfonium salt [24]. In general, although free-radical photo-initiators are more diffused in ceramic resins production because of their attitude with acrylic bases which are easier to use for a rheological motivation, cationic photo-initiators do not need an inert atmosphere, because there are no risks of process inhibition due to oxygen [24]. To give the ceramic resin a reduction in viscosity, thinners are used. These, in addition to giving the resin a low viscosity, contribute to the reduction of the refractive index (RI) [15]. Several diluents have been used in rather high fractions to ensure a significant reduction in viscosity. [PERSON] et al. [16] found that a 30 wt.% of thinner in an 80 wt.% alumina suspension leads to a three-fold reduction in viscosity compared to the viscosity of pure suspension without diluent.