What Is Unsaturated Polyester Resin (UPR)? Types & Key Properties
1. What Is Unsaturated Polyester Resin?
Unsaturated Polyester Resin (UPR) is one of the most widely used thermosetting resins in modern industry—accounting for nearly 75% of all thermosetting resins used in everyday applications.
UPR is a high-molecular polymer produced through a polycondensation reaction between dibasic acids and diols, resulting in a material that contains unsaturated double bonds. These bonds allow the resin to cure and crosslink when combined with a curing agent, transforming it from a liquid into a rigid, durable solid.
Thanks to its versatility, competitive cost, and strong performance, UPR is used across construction, automotive, electrical appliances, marine products, industrial components, SMC/BMC composites, and many other industries.
2. Classification of Unsaturated Polyester Resin
UPR can be classified in several ways. Below are the two most common methods:
A. By Chemical Structure
Different chemical structures give the resin different performance characteristics. Common categories include:
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Ortho-phthalic UPR (e.g., 191, 196)
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Iso-phthalic UPR (e.g., 199)
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Terephthalic and Xylene-based UPR (e.g., 2608, 902A3, XM-1, XM-2)
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Bisphenol-A UPR (e.g., 197, 3301, 323)
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Halogenated UPR (for fire resistance)
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Vinyl Ester Resins (superior corrosion resistance and mechanical strength)
B. By Application Performance
Depending on the intended application, UPR can also be grouped into:
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General-purpose resins
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Heat-resistant resins
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Corrosion-resistant resins
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Flame-retardant resins
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Weather-resistant resins
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High-strength resins
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Gel-coat resins
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SMC/BMC-specific resins
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Resins designed for injection, RTM, pultrusion, and other molding processes
3. Key Characteristics of Unsaturated Polyester Resin
Unsaturated Polyester Resin offers a balanced combination of performance and cost, making it suitable for a wide range of industrial uses. Its main features include:
(1) Heat Resistance
Most UPR grades have heat distortion temperatures of 50–60°C, while specially formulated heat-resistant types can reach up to 120°C.
(2) Mechanical Strength
UPR provides strong tensile, flexural, and compressive strength, making it suitable for structural components and reinforced products.
(3) Chemical Resistance
It performs well against water, diluted acids, and diluted alkalis, though it has poorer resistance to organic solvents.
The level of chemical resistance varies depending on the resin’s molecular structure.
(4) Electrical Insulation Performance
UPR has good dielectric properties, making it suitable for electrical and electronic applications.
(5) Thermosetting Behavior
Once cured, UPR transforms from a soluble, fusible state into an insoluble, infusible solid, providing long-term durability and stability.
(6) Good Processing Flowability
UPR is soluble in suitable solvents and becomes highly flowable when heated, making it easy to mold and compatible with automated manufacturing.
