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Isophthalic Polyester: Structure, Properties, and Applications

In today’s composite materials industry, isophthalic polyester resin is widely recognized for its exceptional performance. It is not only known for its high strength, chemical resistance, and durability, but also for its suitability across various demanding environments.
This article will thoroughly explore the structure, properties, and broad applications of isophthalic polyester in different industries. Continue reading to learn more.

What Is Isophthalic Polyester?

Isophthalic polyester is a type of polyester resin that outperforms other polyester resins such as orthophthalic polyester, particularly in the composites industry. It is renowned for its high strength, chemical resistance, and durability, making it suitable for various demanding applications.

How Is Isophthalic Polyester Produced?

Isophthalic polyester is produced through a series of chemical reactions involving isophthalic acid and other components like ethylene glycol. The production process may vary slightly depending on the specific performance required for the final product, but it generally involves typical polyester synthesis methods, including esterification and polycondensation.

Production Process Overview

  1. Esterification:
    • Initially, isophthalic acid reacts with ethylene glycol to form a prepolymer. This reaction typically occurs under heated conditions and requires water removal (a by-product) to drive the reaction forward.
    • In some processes, catalysts such as dibutyl tin oxide are used to facilitate the reaction.
  2. Polycondensation:
    • The prepolymer then undergoes further polycondensation, reacting with more isophthalic acid and other dicarboxylic acids or their anhydrides to increase molecular weight and form the final polyester structure.
    • This step often requires maintaining high temperatures (around 180°C to 240°C) and may involve staged temperature adjustments to optimize reaction kinetics and resin performance.
  3. Adding Unsaturated Components:
    • For unsaturated polyester resins, unsaturated dicarboxylic acids or anhydrides are added during synthesis to provide sites for subsequent cross-linking.
  4. Final Adjustments and Curing:
    • The resulting polyester resin can be modified by adding various additives to enhance its performance, such as UV stability, flame retardancy, or chemical resistance.
    • Resins are typically cured by adding cross-linking agents like styrene and catalysts that initiate free radical polymerization reactions when heated or exposed to appropriate conditions.

Specific Methods and Variations

  • Some methods involve a single-stage process where all reactants are charged together and react simultaneously.
  • Other methods might use a two-stage process, first reacting the acid with ethylene glycol to form a prepolymer, then further reacting it with other acids and possibly other components like cross-linkers.

What Are the Properties of Isophthalic Polyester?

Isophthalic polyester resin has a range of properties that make it suitable for various demanding applications, particularly where strength, durability, and resistance to environmental factors are required. Here are the main properties based on the source material:

  1. Chemical Resistance:
    • Isophthalic polyester resin exhibits good resistance to a variety of chemicals, including acids, bases, and solvents. This property makes it very suitable for applications that require chemical corrosion resistance or contact with corrosive substances.
  2. Water Resistance:
  3. Heat Resistance:
    • Isophthalic polyester resin has moderate heat resistance, capable of maintaining performance at elevated temperatures.
  4. Low Shrinkage:
    • It typically exhibits low shrinkage during the curing process, which helps minimize the generation of internal stresses in composite materials, enhancing dimensional stability and reducing the likelihood of cracking or warping.
  5. UV Stability:
    • The resin has a moderate level of resistance to UV degradation. However, prolonged exposure to direct sunlight can cause fading and surface degradation. To enhance UV resistance, it is recommended that composite material components exposed outdoors or in environments with strong UV light use UV-resistant coatings or topcoats.
  6. Mechanical Performance:
    • Isophthalic polyester resin possesses excellent mechanical properties, including high tensile strength, impact resistance, and dimensional stability. These characteristics are crucial for applications requiring high strength and durability.
  7. Compatibility with Reinforcement Materials:
    • The resin is compatible with various reinforcement materials, such as glass fibers, carbon fibers, and aramid fibers (like Kevlar). This compatibility allows it to be used with various types of reinforcement materials to manufacture composites with specific mechanical properties and performance.
  8. Curing Process:
    • Isophthalic polyester resin requires catalysts or curing agents to initiate the curing process. If used outside of molds, air inhibitors such as styrene wax or Duratec high gloss additives are needed.

What Are the Common Applications of Isophthalic Polyester?

Isophthalic polyester Resin

Isophthalic polyester resin, with its superior water and chemical resistance compared to orthophthalic resin, is widely used in various fields. Here are some common applications of isophthalic polyester:

  • Glass Fiber Reinforced Plastic (FRP) Products: It is commonly used in the manufacture of glass fiber reinforced plastic (FRP) products that require high chemical resistance, such as pipes and tanks used in the chemical processing industry.
  • Corrosion-Resistant Coatings: Isophthalic polyester coatings are used in automotive, marine, tank, and pipeline chemical service applications to provide corrosion resistance. This high-quality surface protection gel or lining is especially important in environments where materials are exposed to corrosive substances.
  • Marine Industry: Due to isophthalic polyester resin’s high resistance to chemical corrosion, heat, and moisture, and its enhanced tensile properties, it is a feasible solution for manufacturing gel isolation coatings in marine applications. It is also used in the manufacturing process of chemical tanks (including underground oil tanks), thus playing a significant role in the construction and maintenance of the marine industry.
  • Automotive and Transportation: Isophthalic polyester resin is used in automotive parts and wire insulation that require high temperature resistance. Its properties make it suitable for parts that need to withstand high temperatures and mechanical stress.
  • Construction and Infrastructure: This resin is used in cast-in-place cured pipe applications and polyester concrete overlays (PCO), typically based on isophthalic acid. These applications benefit from the resin’s water resistance, chemical resistance, and weatherability, making it suitable for roads, bridges, and other infrastructure projects.
  • Leisure Consumer Products: Isophthalic polyester resin is used in leisure consumer products, where its durability and resistance to environmental factors are highly beneficial.
  • Industrial/Corrosion Protection Applications: Isothalic polyester resin is used in industrial environments for corrosion protection equipment and structures, demonstrating its versatility across various fields.

What Are the Types of Isophthalic Polyester?

Types of isophthalic polyester include:

  • Unsaturated Isophthalic Polyester Resins: This type is commonly used in the composites or glass fiber industry for applications requiring high strength, flexibility, and chemical resistance. It is suitable for traditional laminating, molding, and casting systems.
  • NPG (Neopentyl Glycol) Containing Isophthalic Polyester Resins: This variant is used in high-quality applications such as swimming pool shells. Compared to orthophthalic polyester resin, it has lower sensitivity to hydrolysis and lower shrinkage, making it very suitable for environments with high moisture resistance requirements.

What Are the Benefits of Using Isophthalic Polyester?

The advantages of using isophthalic polyester resin include:

  1. Higher Tensile Strength: Compared to standard polyester resins, isophthalic polyester resin has higher tensile strength, making it more suitable for applications where the material needs to withstand tremendous pressure and strain.
  2. Stronger Chemical Resistance: Its chemical resistance is significantly enhanced, making it very suitable for use in environments where contact with irritant chemicals occurs. This property is particularly applicable in the chemical processing industry or products that come into contact with corrosive substances.
  3. Better Thermal Stability: Isophthalic polyester resin has better thermal stability, able to maintain its integrity and performance at higher temperatures. This makes it suitable for applications where materials are subjected to heat.
  4. Low Shrinkage: This resin has low shrinkage properties, helping to maintain the dimensional stability of the final product. This characteristic is crucial for applications where precision and accuracy in size and shape are required.
  5. Ease of Curing: Isophthalic polyester is easy to cure at room temperature or under heated conditions, providing flexibility in the manufacturing process. This ease of curing helps reduce production time and energy costs.
  6. High Heat Resistance: It can withstand a wide temperature range, from -200°C to up to 270°C. This high heat resistance makes it suitable for applications with drastic temperature changes.
  7. Lightweight: Compared to other materials like phenolic or polyurethane, isophthalic polyester resin is lighter, which is beneficial in applications where weight reduction is needed, such as in the aerospace or automotive industries.
  8. Long Service Life: Due to its durability and wear resistance, isophthalic polyester resin often has a long service life, making it a cost-effective choice for long-term applications.
  9. Widespread Application: Isophthalic polyester resin is widely used, including but not limited to glass fiber manufacturing, shipbuilding, automotive parts, and building materials. Compared to other types of resins, isophthalic polyester resin has superior performance, thus having a broad range of uses.

What Is the Difference Between Isophthalic and Orthophthalic?

The differences between isophthalic and orthophthalic polyester resins lie in their chemical structures, properties, and applications:

  1. Chemical Structure and Components:
    • Isophthalic polyester resin is produced by reacting isophthalic acid with diols, while orthophthalic polyester resin is produced by reacting orthophthalic acid with diols. Isophthalic polyester resin contains isophthalic acid, which gives it a more compact and robust molecular structure. This structural difference affects its performance and applications.
  2. Mechanical Properties and Chemical Resistance:
    • Compared to orthophthalic polyester resin, isophthalic polyester resin has higher mechanical properties and chemical resistance. The more aromatic rings in the molecular structure of isophthalic polyester resin, the higher its degree of cross-linking and chemical resistance. Isophthalic polyester resin is less sensitive to hydrolysis and has a lower shrinkage rate, making it suitable for high-quality applications.
  3. Heat Resistance:
    • Compared to orthophthalic polyester resin, isophthalic polyester resin has a higher melting point and glass transition temperature, providing better heat resistance. This makes isophthalic polyester resin an ideal choice for applications requiring high heat resistance.
  4. Applications:
    • Isophthalic polyester resin is commonly used in applications requiring high chemical corrosion resistance, such as pipes, tanks, chemical storage containers, and marine applications. It is also used in constructing swimming pools, offering excellent water resistance and UV radiation resistance.
    • Orthophthalic polyester resin is less expensive but has lower mechanical strength and chemical resistance, primarily used in applications where chemical resistance and exposure to harsh environments are not highly required. It is typically used in manufacturing glass fiber products with moderate mechanical performance requirements, such as boat parts, automotive parts, and electrical casings.
  5. Cost:
    • Orthophthalic polyester resin is less expensive than isophthalic polyester resin, making orthophthalic polyester resin a more cost-effective choice in applications where the superior performance of isophthalic polyester resin is not required.


Isophthalic polyester resin offers a robust and versatile material option with a wide range of applications due to its superior properties and performance compared to orthophthalic polyester resin. Its high strength, durability, and resistance to harsh environmental conditions make it an ideal choice for many industrial and consumer applications, ensuring long-lasting performance and efficiency.

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