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Raw Materials and Classification of Saturated Polyester Resins

Release time:

2024-07-15 16:16

Raw Materials and Classification of Saturated Polyester Resins
Raw Materials and Classification of Saturated Polyester Resins

Saturated polyester resin is a polymer compound in which all the carbon-carbon bonds in the main chain are saturated, meaning they do not contain any unsaturated double bonds. This type of resin is typically produced via a polycondensation reaction between diols and dicarboxylic acids and exhibits excellent physical and chemical properties. Depending on the raw materials and synthesis methods used, saturated polyester resins can be classified into several types, such as linear polymers, branched polymers, and modified polyester resins.

I. Raw Materials

In the synthesis of saturated polyester resins, the core raw materials include diols, dicarboxylic acids, and triols; occasionally, monohydric alcohols or monocarboxylic acids are also added to fine-tune the resin’s properties. Among the various diols, neopentyl glycol has become the primary material due to its esterification products exhibiting outstanding water resistance—a property significantly superior to that of ethylene glycol and propylene glycol. As for triols, trimethylolpropane and trimethylolethane play a crucial role in enhancing the resin’s crosslinking degree and mechanical performance thanks to their unique chemical structures.

1. Diol

Diacids are one of the fundamental raw materials for synthesizing polyester resins. They form polyester chains by undergoing esterification reactions with dicarboxylic acids. Neopentyl glycol, thanks to its unique molecular structure, produces esterified products with outstanding water resistance—far superior to those obtained from ethylene glycol and propylene glycol. This characteristic makes neopentyl glycol the material of choice when synthesizing polyester resins that require high water resistance.

2. Trihydric alcohol

The introduction of triols creates branching points in the polyester resin molecular chains, which helps increase the resin’s crosslinking density and mechanical properties. Trimethylolpropane and trimethylolethane are commonly used triols. Their use allows adjustment of the resin’s hardness and flexibility, thereby meeting the requirements of various applications.

3. Dibasic acid

a. Aromatic dicarboxylic acid

Isophthalic acid occupies an important position in the synthesis of polyester resins due to its outstanding resistance to salt spray, chemical resistance, and water resistance. Compared with orthophthalic acid, isophthalic acid imparts greater environmental adaptability and durability to the resin.

b. Aliphatic dicarboxylic acid

Aliphatic dicarboxylic acids such as adipic acid, azelaic acid, and sebacic acid are also commonly used in the synthesis of polyester resins. They primarily affect the resin’s flexibility and processability; among them, adipic acid is the most widely used due to its cost-effectiveness and well-balanced performance.

II. The Influence of Raw Materials on Performance

When synthesizing polyester resins, the glass transition temperature (Tg) of the resin can be precisely controlled by adjusting the molar ratio of aromatic dicarboxylic acids to aliphatic dicarboxylic acids. Tg is an important physical property indicator for polymeric materials, as it determines the material's operating temperature range, hardness, elasticity, and other characteristics. Generally speaking, increasing the proportion of aromatic dicarboxylic acids raises the resin's Tg, endowing it with higher heat resistance and hardness; conversely, increasing the proportion of aliphatic dicarboxylic acids makes the resin softer and enhances its processability and flexibility.

III. Classification

1. Linear polymer

Definition: A linear polymer is a saturated polyester resin with a linear molecular structure, synthesized via a polycondensation reaction between linear-chain polyols and polybasic acids (which may include monobasic acids).

Characteristics: These resins typically exhibit good flexibility and adhesion, making them suitable for applications that require high flexibility and adhesion.

Applications: In the coatings industry, linear polymers are commonly used in fields such as coil coatings, industrial coatings, automotive paints, and powder coatings.

2. Branched Polymers

Definition: Branched polymers are synthesized via polycondensation reactions using polyols and polyacids with multiple functional groups, and their molecular structure contains branched chains.

Characteristics: Compared to linear polymers, branched polymers may exhibit higher cross-link density and better chemical resistance, but their flexibility might be somewhat reduced.

Applications: Branched polymers are widely used in fields that require high hardness and chemical resistance, such as certain specialty coatings and adhesives.

3. Modified polyester resin

Definition: Modified polyester resins are products whose properties are altered during synthesis by introducing components other than polyols and polyacids (such as epoxies, acrylates, organosilicon compounds, etc.).

Features: Modified polyester resins can achieve properties that original polyester resins do not possess, such as higher hardness, weather resistance, chemical resistance, or improved processability, depending on the specific requirements.

Applications: Modified polyester resins are widely used in various fields such as coatings, inks, and adhesives—particularly in applications that demand specific resin performance characteristics.

IV. Summary

Saturated polyester resins can be classified into various types depending on their raw materials and synthesis methods. Each type has its own unique properties and application scenarios, allowing users to select the appropriate type based on their specific needs. In practical applications, it is also necessary to consider how factors such as molecular weight, functionality, and cross-linking density of the resin affect its performance.

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