Manufacturing of Polyester by TPA/PTA Route – Direct Esterification Process

Introduction

Polyester is a widely used man-made fibre. One important method of manufacturing polyester is the TPA/PTA route, also called the direct esterification (DE) process.

In this process, terephthalic acid (TPA) reacts directly with monoethylene glycol (MEG) to form an esterified intermediate. Water is produced as a by-product and is removed from the reaction system. The intermediate is then converted into high-molecular-weight polyethylene terephthalate (PET) by polycondensation.

The PTA route is an alternative to the DMT route. Its main difference is the use of terephthalic acid instead of dimethyl terephthalate as the starting material.

1. What are TPA and PTA?

• TPA (Terephthalic Acid): The acid used as a raw material for manufacturing PET.
• PTA (Purified Terephthalic Acid): Terephthalic acid purified to a quality suitable for fibre production.

2. Raw Materials Used

The two main raw materials are:

Raw material	Function
Terephthalic acid (TPA/PTA)	Provides the terephthalate component for PET formation
Monoethylene glycol (MEG)	Reacts with TPA to form the esterified intermediate

Prepare a mixture of TPA and MEG in a proportion of approximately 1:1 to 1:1.3.

3. Flowchart of Polyester Manufacturing by TPA/PTA Route

The overall route consists of direct esterification followed by polycondensation. The polycondensation stage is common to both the DMT and PTA processes.

4. Manufacturing Process of Polyester by TPA/PTA Route

Step 1: Preparation of Raw Materials

The main raw materials, terephthalic acid and monoethylene glycol, are mixed before the direct esterification reaction. TPA has low solubility in MEG.

• Temperature: 240–260°C
• Pressure: 400 kPa
• TPA:MEG proportion: 1:1 to 1:1.3

Step 2: Direct Esterification

The prepared TPA and MEG mixture is heated under pressure to carry out the direct esterification reaction. During this stage:

1. TPA reacts directly with MEG.
2. Ester linkages are formed.
3. An esterified intermediate is produced.
4. Water is formed as a by-product.

TPA itself accelerates the direct esterification reaction. If required, stronger acids or esters of titanic acid may be used as additional catalysts.

Step 3: Removal of Water

Water formed during direct esterification is allowed to distil out of the reaction system. Unlike the DMT route, the PTA route does not produce methanol during this stage. Therefore, the methanol reflux arrangement used in the DMT process is not required.

Step 4: Addition of Thermal Stabilizer

After direct esterification is completed, a thermal stabilizer is added to the low-viscosity melt. The material is then transferred to the polycondensation vessel.

Step 5: Melt Polycondensation

The esterified intermediate is transferred to the polycondensation vessel for further polymerisation. During this stage:

1. The intermediate undergoes polycondensation.
2. PET molecular chains grow.
3. Excess MEG is removed from the reaction system.
4. The molecular weight of the polymer increases.

The polycondensation stage is common to both DMT and PTA routes. A second dose of catalyst may be added if required.

Step 6: Vacuum Application and Molecular Weight Build-up

The final molecular weight development is diffusion-controlled rather than chemically controlled. Therefore, the pressure is progressively reduced during polycondensation. This helps the process achieve a high-molecular-weight polymer.

Step 7: Recovery of Excess MEG

Excess MEG is removed during polymer production and recovered for recycling.

Step 8: PET Melt Production

After polycondensation, high-molecular-weight PET melt is obtained. The polymer melt can then be sent for fibre production.

Step 9: Spinning and Fibre Processing

The subsequent fibre-processing operations, including:

• Spinning
• Spin finishing
• Drawing and twisting for filament yarn
• Crimping, drying and cutting for staple fibre

These operations convert the PET polymer into polyester filament yarn or staple fibre.

5. Catalysts Used in the PTA Route

The PTA route has a different catalyst requirement from the DMT route.

• TPA itself accelerates the direct esterification reaction.
• Stronger acids may be added when required.
• Esters of titanic acid may also be used as additional catalysts.
• A second dose of catalyst may be added during polycondensation if necessary.

The absence of a metal catalyst in the PTA route prevents certain metal-catalysed degradation reactions that can occur in the DMT route. However, some PET degradation can still occur during polycondensation because the thermal decomposition temperature of PET is close to the temperature used in that stage.

6. Side Reactions in the PTA Route

One important side reaction is the formation of diethylene glycol (DEG). The ether formation is greater during direct esterification than during the ester-interchange process used in the DMT route. This side reaction can be suppressed by:

• Adding a small amount of sodium hydroxide (NaOH).
• Adding an organic quaternary hydroxide.
• Using a higher temperature, 280–290°C.

7. Properties of PET Produced by the PTA Route

PET produced by the PTA route generally has the following characteristics compared with PET produced by the DMT route:

Property	PTA-route PET compared with DMT-route PET
Average molecular weight	Lower
Melting point	Lower
Dye uptake	Higher
DEG content	Higher

These differences are generally attributed to the higher DEG content in PET produced by the PTA process.

8. Advantages of the PTA Route

1. Lower raw-material cost: TPA is usually less expensive than DMT.
2. No methanol formation: Methanol is not produced during the esterification stage.
3. Self-catalysed reaction: TPA itself accelerates direct esterification.
4. Superior product quality: Superior product quality as an advantage of this route.
5. Higher molecular weight of the esterification product: The reaction product from direct esterification has a higher molecular weight than the product obtained through the ester-interchange step in the DMT route.

9. Difference Between DMT Route and TPA/PTA Route

Both processes are used to manufacture PET. The main difference is the starting material and the first reaction stage.

Comparison point	DMT route	TPA/PTA route
Main raw material	Dimethyl terephthalate (DMT)	Terephthalic acid (TPA/PTA)
First reaction	Ester interchange or transesterification	Direct esterification
Other main reactant	MEG	MEG
Main by-product in first stage	Methanol	Water
Reaction catalyst	Ester-interchange catalyst	TPA itself accelerates the reaction; additional catalysts may be used
Reflux arrangement	Methanol reflux arrangement is used	Methanol reflux arrangement is not required
Esterification product	Lower-molecular-weight product	Higher-molecular-weight product
Polycondensation	Required	Required
Final polymer	PET	PET

10. Important Terms

Term	Simple explanation
TPA	Terephthalic acid used in PET manufacture
PTA	Purified terephthalic acid suitable for fibre production
MEG	Monoethylene glycol, which reacts with TPA
Direct esterification	Reaction of an acid with a glycol to form an esterified product
Polycondensation	Further reaction in which polymer chains grow
DEG	Diethylene glycol, formed through a side reaction
Thermal stabilizer	An additive introduced to the melt before polycondensation in the described process
Vacuum	Reduced pressure used during polycondensation to support molecular-weight build-up

11. Frequently Asked Questions (FAQs)

1. What is the TPA/PTA route of polyester manufacturing?

The TPA/PTA route is a method of manufacturing PET by reacting terephthalic acid directly with monoethylene glycol. It is also called the direct esterification process.

2. What are the raw materials used in the PTA route?

The two main raw materials are purified terephthalic acid (PTA) and monoethylene glycol (MEG).

3. What is the main reaction in the PTA route?

The main reaction is direct esterification between TPA and MEG, producing an esterified intermediate and water.

4. Which by-product is formed during direct esterification?

Water is formed as a by-product during direct esterification.

5. Is methanol produced in the PTA route?

No. Methanol is not produced during the direct esterification stage of the PTA route. Methanol is associated with the ester-interchange stage of the DMT route.

6. Which catalysts are used in the PTA route?

TPA itself accelerates the direct esterification reaction. Stronger acids or esters of titanic acid may also be used as additional catalysts.

7. Why is vacuum used during polycondensation?

Final molecular-weight development is diffusion-controlled. Progressive reduction of pressure helps achieve high-molecular-weight PET.

8. What is the main side reaction in the PTA route?

One important side reaction is the formation of diethylene glycol (DEG).

9. How does PTA-route PET differ from DMT-route PET?

PTA-route PET generally has lower average molecular weight, lower melting point and higher dye uptake than DMT-route PET.

10. Is polycondensation different in the DMT and PTA routes?

No. The polycondensation stage as common to both routes.