1. How to Select Dicarboxylic Acids for Polyester Polyol Formulation?

Aromatic Acids: Improving Rigidity and Heat Resistance

Isophthalic Acid (IPA) and Terephthalic Acid (PTA)

Aromatic dicarboxylic acids are mainly used to improve:

Terephthalic acid (PTA) has a highly regular linear structure and strong crystallization tendency.

When PTA reacts with short-chain diols such as ethylene glycol (EG) or 1,4-butanediol (BDO), the resulting polyester structure may show strong crystallization behavior, causing the polyester polyol to become waxy or solid at room temperature.

This may reduce:

• ✖ Processing fluidity
• ✖ Storage stability
• ✖ Application convenience

Therefore, pure PTA-based polyester polyols are rarely used alone in practical formulations.

Isophthalic acid (IPA) has a less symmetrical molecular structure because the two carboxyl groups are located at the meta position. This reduces crystallization tendency and improves liquid stability.

IPA-based polyester polyols generally provide:

• ✔ Higher Tg
• ✔ Better transparency
• ✔ Improved flowability

In many industrial formulations, IPA is introduced together with PTA or aliphatic acids to balance rigidity and flexibility.

Aliphatic Acids: Increasing Flexibility

Adipic Acid (AA)

Adipic acid is one of the most commonly used aliphatic dicarboxylic acids in polyester polyol production.

Its flexible six-carbon chain provides:

• • Low-temperature flexibility
• • Good elasticity
• • Reduced Tg

Adipic acid-based polyester polyols usually have Tg values in the range of approximately -60°C to -40°C, depending on molecular structure.

This structure contributes to:

• • High tensile strength
• • Good abrasion resistance
• • Excellent mechanical performance

Sebacic Acid (SA)

Sebacic acid contains a longer carbon chain than adipic acid.

Advantages:

• ✔ Higher flexibility
• ✔ Better low-temperature performance
• ✔ Improved hydrophobic properties

However, due to its higher cost, SA is mainly used in high-performance polyurethane applications where enhanced cold resistance is required.

Functional Acid Components: Dimer Acid and Phthalic Anhydride

Dimer Acid

Dimer acid introduces:

• • Long flexible molecular chains
• • Hydrophobic properties
• • Improved flexibility
• • Better hydrolysis resistance

However, increasing dimer acid content may reduce Tg and hardness. It is commonly considered for polyurethane systems requiring flexibility and moisture resistance.

Phthalic Anhydride (PA)

Phthalic anhydride is a cost-effective aromatic component with good reaction efficiency.

It can improve:

• ✔ Hardness
• ✔ Rigidity
• ✔ Cost competitiveness

However, due to steric effects, its terminal group reactivity is lower compared with IPA.

Typical Acid Design Strategy

A common polyester polyol formulation strategy is:

• • Aromatic acids: provide rigidity and strength
• • Aliphatic acids: provide flexibility and toughness

The exact ratio should always be adjusted according to the target application.

2. How to Select Diols for Polyester Polyol Design?

Ethylene Glycol (EG)

Ethylene glycol is the simplest diol used in polyester chemistry.

Because of its short molecular structure, EG provides:

• • High chain regularity
• • Strong crystallization tendency

When combined with PTA, EG can form PET-like crystalline structures, which may cause polyester polyols to become solid at room temperature. Therefore, pure EG-based polyester polyols are uncommon.

In practical formulations, EG is often combined with:

to adjust crystallinity and processing performance.

One important advantage of EG is its:

• ✔ Low cost
• ✔ Wide availability

For polyester polyol production based on recycled PET materials, EG remains an important raw material.

Diethylene Glycol (DEG)

DEG introduces ether bonds into the polyester chain.

The flexible ether linkage provides:

• • Lower Tg
• • Reduced viscosity
• • Better flow performance

DEG-based polyester polyols are widely used when processing flexibility is important.

However, ether bonds may have lower resistance to long-term thermal oxidation. Therefore, DEG systems are often combined with more stable diols such as NPG.

Neopentyl Glycol (NPG)

NPG contains a quaternary carbon structure with two methyl groups.

This unique structure restricts molecular movement and provides:

• • Higher Tg
• • Excellent thermal stability
• • Superior weather resistance
• • Improved hydrolysis resistance

Because of its excellent durability, NPG is widely used in:

• • PU coatings
• • Outdoor applications
• • Weather-resistant polyurethane systems

1,4-Butanediol (BDO)

1,4-Butanediol is one of the most important diols used in polyurethane elastomer and TPU applications.

Its four-carbon linear structure provides:

• • Good chain regularity
• • Strong intermolecular interaction
• • Excellent mechanical properties

When combined with aromatic acids such as PTA, BDO can form highly ordered crystalline segments similar to PBT structures.

This contributes to:

• ✔ Higher tensile strength
• ✔ Better abrasion resistance
• ✔ Improved physical crosslinking

However, high BDO content may increase crystallization tendency, causing polyester polyols to become waxy or solid at room temperature.

To improve processing performance, BDO is often combined with:

to reduce excessive crystallization while maintaining mechanical strength.

Trimethylolpropane (TMP): Introducing Controlled Branching

Trimethylolpropane (TMP) is a multifunctional alcohol containing three hydroxyl groups. Unlike conventional diols, TMP introduces branching structures into polyester polyols.

A small amount of TMP can increase:

• • Molecular branching
• • Crosslink density
• • Final polyurethane hardness

TMP is commonly used when designing:

• • High-performance polyurethane coatings
• • Rigid PU systems
• • Branched polyurethane prepolymers

In most formulations, TMP content is carefully controlled because excessive branching may significantly increase viscosity and reduce processability.

3. How to Adjust the Alcohol-to-Acid Ratio in Polyester Polyol Formulation?

Alcohol Excess Ratio

In polyester polyol synthesis, alcohol components are usually used in excess to achieve hydroxyl-terminated structures.

For lower molecular weight polyester polyols, the alcohol excess ratio may be increased to: 1.8–2.0 : 1

Higher hydroxyl values provide more reactive hydroxyl groups for polyurethane formation.

However, excessive alcohol content may reduce molecular weight and negatively affect:

Therefore, the alcohol-to-acid ratio must be optimized according to the final polyurethane application.

Relationship Between Alcohol Ratio and Acid Value

During polyester polyol synthesis, generated water is continuously removed to drive esterification and condensation reactions forward.

If the alcohol excess is insufficient:

• • Acid concentration remains high
• • Reaction becomes slower
• • Final acid value becomes difficult to reduce

A low acid value is important because residual acids may affect polyurethane reactions. When polyester polyol reacts with isocyanates, residual moisture and acidic components may contribute to:

For many polyurethane applications, controlling acid value below: 1 mg KOH/g is an important quality target.

4. How to Select Catalysts for Polyester Polyol Production?

The selection between TBT and BTO depends on:

5. Polyester Polyol Manufacturing Process Control

The main stages include:

1. Esterification
2. Polycondensation
3. Hydroxyl value and acid value adjustment

Step 1: Esterification Stage

During the esterification stage, the following materials are charged into the reactor:

The reaction usually begins at approximately: 180°C

Water generated from esterification is removed through:

• • Nitrogen stripping
• • Condenser systems

Reaction progress can be monitored through:

• • Water collection amount
• • Acid value testing

Step 2: Polycondensation Stage

After esterification, the system enters the polycondensation stage.

Typical conditions:

• • Temperature: 200–220°C
• • Reduced pressure: vacuum operation

Vacuum helps remove remaining water and excess alcohol, driving molecular growth.

However, excessive temperature or prolonged reaction time may cause side reactions, color increase, and polymer degradation. Therefore, process conditions must be carefully controlled.

Step 3: End Point Control

Unlike PET polymerization, polyester polyol production does not require extremely high molecular weight development. The main quality indicators are:

A common industrial target: Acid value <1 mg KOH/g before discharge.

Insufficient vacuum is one of the major reasons for:

• ✖ High residual moisture
• ✖ Higher acid value
• ✖ Poor polyurethane performance

6. Polyester Polyol Design Summary

Design Factor	Key Variables	Selection Principle	Typical Range
Backbone rigidity	IPA, PTA, AA, SA	Aromatic acids improve rigidity; aliphatic acids improve flexibility	IPA/AA ≈ 7:3
Tg adjustment	EG, DEG, NPG, BDO	EG increases crystallinity; DEG reduces viscosity; NPG improves durability; BDO improves strength	Based on application
Molecular weight control	Alcohol-to-acid ratio	Alcohol excess creates hydroxyl-terminated structures	Alcohol/Acid = 1.3
Catalyst selection	TBT vs BTO	TBT for fast reaction; BTO for better hydrolysis stability	BTO 0.2–0.5‰
Branching control	TMP/Glycerol	Small additions improve crosslink density	TMP ≤3 mol%

7. How Polyester Polyol Design Affects Polyurethane Performance?

Application	Recommended Polyester Polyol Design
PU Adhesives	Flexible AA-based polyester polyols for strong adhesion and toughness
PU Coatings	IPA/NPG systems for hardness and weather resistance
TPU Elastomers	AA + BDO systems for mechanical strength and abrasion resistance
Waterborne PU	Hydrolysis-resistant polyester polyols with optimized catalyst systems
Synthetic Leather	Flexible polyester polyols with controlled softness

Application requirements → Performance targets → Monomer selection → Molecular structure design → Catalyst selection → Process optimization