Unsaturated polyester is a linear polymer compound formed by the condensation of unsaturated dicarboxylic acids (or anhydrides) or mixed acids composed of unsaturated dicarboxylic acids (or anhydrides) and polyols, with ester bonds and unsaturated double bonds. Usually, the polyester polymerization reaction is carried out at 190-220 ℃ until the expected acid value (or viscosity) is reached. After the polyester shrinkage reaction is completed, a certain amount of vinyl monomer is added while hot to form a viscous liquid. This polymer solution is called unsaturated polyester resin.
1, Physical and chemical properties
1. Physical properties. The relative density of unsaturated polyester resin is around 1.11 to 1.20, and the volume shrinkage rate is relatively large during curing. Some physical properties of cured resin are as follows: ⑴ Heat resistance. The thermal deformation temperature of the vast majority of unsaturated polyester resins is between 50-60 ℃, while some resins with good heat resistance can reach up to 120 ℃. The coefficient of thermal expansion α 1 is (130-150) × 10-6 ℃. ⑵ Mechanical properties. Unsaturated polyester resin has high tensile, bending, compression and other strength. ⑶ Chemical corrosion resistance. Unsaturated polyester resin has good resistance to water, dilute acid, and dilute alkali, but poor resistance to organic solvents. At the same time, the chemical corrosion resistance of the resin can vary greatly depending on its chemical structure and geometric switch. ⑷ Dielectric properties. Unsaturated polyacid resin has good dielectric properties.
2. Chemical properties. Unsaturated polyester is a linear polymer compound with multifunctional groups, which has polyester chain bonds and unsaturated double bonds on its backbone main chain, and carboxyl and hydroxyl groups at both ends of the polymer chain. The double bonds on the main chain can undergo copolymerization crosslinking reaction with vinyl monomers, causing unsaturated polyester resins to transition from soluble and meltable states to insoluble and non melting states. The ester bonds on the main chain can undergo hydrolysis reactions, which can be accelerated by acids or bases. If crosslinked with styrene, the occurrence of hydrolysis reaction can be greatly reduced. In acidic media, hydrolysis is reversible and incomplete, so polyester can withstand the erosion of acidic media; In alkaline media, due to the formation of resonance stable carboxylate anions, hydrolysis becomes irreversible, resulting in poor alkali resistance of polyester. The carboxyl group at the end of the polyester chain can react with alkaline earth metal oxides or hydroxides [such as MgO, CaO, Ca (OH) 2, etc.] to expand the unsaturated polyester molecular chain, which may eventually form complexes. The molecular chain expansion can make the initial viscosity of viscous liquid resin be 0.1~1.0Pa? S, and the viscosity will increase sharply to over 103Pa? S in a short time, until it becomes a similar gel like substance that cannot flow and does not stick to hands. When the resin is in this state, it is not crosslinked and can still dissolve in suitable solvents. It has good fluidity when heated
