Unsaturated polyester resin (UPR), also known as unsaturated polyester, is a type of thermosetting resin characterized by its linear polymer structure containing unsaturated double bonds in the main chain. This unique chemical structure allows UPR to undergo polymerization reactions with vinyl monomers such as styrene, acrylates, and vinyl acetate under the influence of initiators and accelerators, ultimately transforming into an insoluble and infusible high-molecular-weight network polymer. This versatile material has found widespread applications across various industries due to its exceptional properties and processing capabilities.

The Chemical Structure and Synthesis

UPR is synthesized through the polycondensation reaction between unsaturated dibasic acids (or anhydrides) and saturated dibasic acids with diols. Typically, the raw materials include 1,2-propanediol, maleic anhydride, and phthalic anhydride, which are mixed in specific molar ratios and polymerized under nitrogen or carbon dioxide atmospheres at temperatures ranging from 150 to 200°C for several hours. This process results in a polyester with a molecular weight of 1000 to 2500 and a low acid value.

After cooling, the polyester is diluted with styrene containing a hydroquinone inhibitor, typically in amounts 1.5 to 2.0 times the moles of unsaturated acid. The styrene not only acts as a solvent but also participates in the crosslinking reaction during the curing process. The resulting UPR can be stored at room temperature for several months to a year.

Properties and Characteristics

UPR boasts a range of properties that make it suitable for diverse applications. Its relative density falls between 1.11 and 1.20, and it experiences significant volume contraction during curing. The thermal deformation temperature of most UPR grades is between 50 and 60°C, but some heat-resistant varieties can reach up to 120°C. UPR exhibits high tensile, bending, and compressive strengths and is resistant to water, diluted acids, and alkalis, although it has poorer resistance to organic solvents.

UPR's electrical properties and flame retardancy can be tailored by selecting appropriate resin grades. The resin's light color allows for the production of transparent products. Furthermore, its variety, adaptability, and relatively low price enhance its appeal. However, UPR has a relatively short shelf life compared to other resins.

Applications

The primary application of UPR is in the production of fiberglass-reinforced plastics (FRPs), also known as glass-reinforced plastics (GRPs) or glass-fiber-reinforced polyester. When reinforced with glass fibers, UPR's mechanical properties are significantly enhanced, resulting in a composite material with high stiffness, strength, and lightweight characteristics. FRP products made from UPR are widely used in the construction of cooling towers, piping, tanks, gratings, and other corrosion-resistant structures. They are also employed in the manufacturing of vehicles, boats, and amusement equipment due to their durability and lightweight properties.

In addition to FRP applications, UPR is also used in non-reinforced forms for products such as coatings, decorative castings, electrical castings, polyester putties, and adhesives. The casting process involves mixing UPR with initiators, accelerators, and fillers to form a flowable material, which is then poured into molds and cured at room temperature or with heat.

Environmental and Safety Considerations

During its processing, UPR releases styrene and other volatile organic compounds, which may pose health risks if not adequately controlled. Proper ventilation and personal protective equipment are essential to minimize exposure. Additionally, the disposal of waste materials containing UPR requires careful management to prevent environmental contamination.

Conclusion

Unsaturated polyester resin is a versatile and widely used thermosetting resin with a unique chemical structure that enables it to undergo polymerization reactions with vinyl monomers. Its range of properties, including high strength, corrosion resistance, and electrical properties, makes it ideal for applications such as FRP production, coatings, and castings. Despite its shortcomings, such as short shelf life and environmental concerns, UPR continues to be a cornerstone material in the manufacturing industry due to its cost-effectiveness and processing ease. As research and technology advance, the applications and capabilities of unsaturated polyester resin are likely to expand further, contributing to the development of innovative and sustainable products across various sectors.