Unsaturated polyester is a type of linear polymer that contains both ester groups and unsaturated double bonds in its main chain. It also has carboxyl and hydroxyl groups at the ends of the polymer chains. This unique structure gives it specific chemical and physical properties that make it useful in various applications.
The presence of unsaturated double bonds allows the resin to undergo crosslinking when mixed with vinyl monomers like styrene. This reaction transforms the material from a soluble, thermoplastic state into an insoluble, thermoset form, which is essential for many composite manufacturing processes.
The ester bonds in the main chain are susceptible to hydrolysis, especially under acidic or basic conditions. However, when the polyester is copolymerized with styrene, this hydrolytic degradation is significantly reduced, improving the material's stability in different environments.
In acidic environments, hydrolysis is typically reversible and limited, allowing the polyester to remain stable. On the other hand, in alkaline conditions, the hydrolysis becomes more severe due to the formation of resonance-stabilized carboxylate anions, making the material less resistant to alkali.
At the ends of the polymer chains, the carboxyl groups can react with metal oxides or hydroxides such as magnesium oxide (MgO) or calcium hydroxide (Ca(OH)₂). This reaction leads to chain extension, increasing the viscosity of the resin. Initially, the resin may have a low viscosity of around 0.1 to 1.0 Pa·s, but it rapidly thickens within a short time, reaching values above 10³ Pa·s and forming a gel-like, non-flowing substance. Despite this, the resin remains soluble in certain solvents and retains good flow characteristics when heated, indicating that it hasn't fully crosslinked yet.
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