1. Introduction

Polyester fibre is manufactured mainly from polyethylene terephthalate (PET). PET is a synthetic polymer produced through a chemical reaction between a terephthalate compound and ethylene glycol.

The DMT route is one of the two methods used for manufacturing PET. In this method, dimethyl terephthalate (DMT) reacts with monoethylene glycol (MEG) through a process called transesterification or ester interchange.

The manufacturing process consists of two major stages:

1. Transesterification: DMT reacts with MEG to form diglycol terephthalate (DGT), with methanol released as a by-product.
2. Polycondensation: DGT molecules react with each other to form high-molecular-weight PET, with MEG released as a by-product.

The resulting PET polymer is used for manufacturing polyester fibres.

2. Raw Materials Used in the DMT Route

Raw material	Full name	Function
DMT	Dimethyl terephthalate	Main terephthalate raw material
MEG	Monoethylene glycol	Reacts with DMT to form the intermediate
Catalyst	Chemical catalyst	Accelerates transesterification
Thermal stabilizer	Stabilising additive	Helps improve the thermal stability of the polymer melt

Important intermediate: Diglycol terephthalate (DGT) is formed during the first stage. It provides a chemically defined intermediate for the subsequent polycondensation reaction.

3. Flowchart of Polyester Manufacturing by DMT Route

4. Chemical Reactions Involved in the DMT Route

4.1 First reaction: Transesterification

In the first stage, DMT reacts with two molecules of MEG. During this reaction, the methoxy groups of DMT are replaced by glycol groups. Methanol is produced and removed from the reaction mixture.

4.2 Second reaction: Polycondensation

In the second stage, DGT molecules undergo polycondensation to produce PET. During this reaction, smaller glycol molecules are eliminated as the polymer chains grow. The main objective is to obtain PET with sufficiently high molecular weight for fibre production.

5. Detailed Manufacturing Process of Polyester by DMT Route

5.1 Melting and preparation of DMT

The first operation is the preparation of DMT in molten form.

The operations are:

1. Solid DMT is fed into a melter.
2. The DMT is heated until it melts.
3. A heavy-duty pump transfers the molten DMT.
4. The molten material passes through filter packs to remove unmelted particles and impurities.
5. The filtered DMT is collected in a heated reservoir.
6. The reservoir maintains the DMT in molten condition before it enters the transesterification reactor.

The reference describes a reservoir temperature of 150°C for maintaining molten DMT.

5.2 Transesterification or ester interchange

First major reaction stage. Transesterification is the reaction between DMT and MEG to form glycol terephthalate intermediates. It is also known as ester interchange (EI).

Purpose:

• To replace the methyl ester groups of DMT with glycol ester groups.
• To form diglycol terephthalate.
• To remove methanol from the reaction system.

Working procedure:

1. Molten DMT is transferred into the transesterification vessel.
2. MEG is added to the reaction mixture.
3. A suitable catalyst is introduced to increase the reaction rate.
4. The mixture is heated and continuously agitated.
5. Methanol is formed during the reaction.
6. Methanol vapour passes through the separation arrangement.
7. The reflux column helps return evaporated MEG to the reactor while allowing methanol to be separated.
8. The reaction mixture gradually changes into glycol terephthalate intermediates and low-molecular-weight oligomers.

Operating conditions: The reference describes continuous transesterification near the boiling point of MEG, approximately 197°C, with a reaction time of around 3–6 hours in the described process. These are reference-specific conditions rather than universal settings for every plant.

5.3 Catalysts used in transesterification

Transesterification between DMT and MEG is relatively slow. Therefore, catalysts are required to accelerate the reaction. The uploaded references discuss several metal-based catalyst systems, including:

• Zinc compounds.
• Manganese compounds.
• Cobalt compounds.
• Lead compounds.
• Cadmium compounds.
• Titanium compounds.

Functions of the catalyst:

• Increases the rate of transesterification.
• Helps the exchange reaction proceed efficiently.
• Supports the formation of glycol terephthalate intermediates.

The exact catalyst composition depends on the manufacturing process.

5.4 Removal of methanol

Methanol removal is an important operation during transesterification. Methanol is produced as a by-product when the methyl ester groups of DMT react with MEG. The removal process involves:

1. Methanol evaporates from the heated reaction mixture.
2. The vapour enters a separation column.
3. The column helps separate methanol from the glycol-containing vapour mixture.
4. MEG is condensed and returned to the reaction vessel.
5. Methanol is collected separately.

Why is methanol removed?

The transesterification reaction is reversible. Removing methanol helps drive the reaction towards the formation of glycol terephthalate intermediates.

5.5 Formation of diglycol terephthalate (DGT)

As transesterification proceeds, DGT is formed. DGT is the intermediate used for the next stage of polyester manufacturing. The intermediate is transferred to the polycondensation section after sufficient conversion. The formation of a chemically defined intermediate provides several advantages:

• More uniform starting material for polycondensation.
• Fewer side reactions.
• Better control of polymer formation.
• Improved polyester colour.
• Better thermal stability of the polymer melt.
• Improved fibre drawability.

5.6 Polycondensation

Polycondensation is the process in which smaller glycol terephthalate molecules combine to form long PET polymer chains. This stage increases the molecular weight of the polymer.

Working procedure:

1. The glycol terephthalate intermediate is transferred into the polycondensation reactor.
2. The reaction mixture is heated to a higher temperature.
3. The pressure inside the reactor is gradually reduced.
4. MEG is produced and removed from the reaction mixture.
5. The removal of MEG promotes further polymer-chain growth.
6. The viscosity of the polymer melt increases as molecular weight increases.
7. The reaction continues until the required polymer characteristics are obtained.

Why is vacuum necessary?

During polycondensation, MEG must be removed efficiently. Reduced pressure helps remove this volatile by-product. The removal of MEG shifts the reaction towards the formation of longer polymer chains. The final product is high-molecular-weight PET.

5.7 Thermal stabilisation

Thermal stabilisers may be added to help improve the stability of the polymer melt during subsequent processing. The uploaded references also discuss the use of phosphorus-containing compounds, including phosphites and phosphates, in connection with polyester melt stabilisation and catalyst control. The specific stabiliser formulation and dosage for the DMT process are not consistently specified in the available reference extracts.

5.8 PET polymer formation and finishing

After polycondensation, the PET melt is transferred for further processing. Depending on the plant arrangement, the polymer may be:

• Extruded and converted into PET chips.
• Transferred directly to spinning machines in an integrated continuous process.

In the chip-based process, the chips are subsequently processed through crystallisation, drying and remelting before melt spinning. In the direct-spinning process, the polymer melt from the final polycondensation vessel is supplied directly to the spinning machines.

6. Important By-products in the DMT Route

Manufacturing stage	By-product	Reason for formation
Transesterification	Methanol	Exchange of methyl ester groups with glycol groups
Polycondensation	MEG	Elimination of glycol during polymer-chain growth

Remember:

• Transesterification removes methanol.
• Polycondensation removes MEG.

This is one of the most important differences to remember when studying the DMT route.

7. Side Reactions and Problems During Polyester Manufacturing

Some unwanted reactions may occur during the DMT process. These reactions can affect the quality and molecular weight of PET.

7.1 Formation of diethylene glycol (DEG)

MEG can undergo a side reaction that produces diethylene glycol. DEG may become incorporated into the PET polymer chain. Excessive formation of DEG can influence the properties of the resulting polyester.

7.2 Thermal degradation

PET is processed at elevated temperatures. Uncontrolled heating and excessive thermal exposure can cause polymer degradation. The uploaded references identify thermal damage of DMT and excessive formation of glycol-related by-products as factors that can affect polymer quality.

7.3 Incomplete transesterification

If the transesterification reaction does not proceed sufficiently, some methyl ester groups remain unconverted. The presence of unconverted groups can restrict the development of high-molecular-weight PET.

7.4 Importance of raw material purity

The purity of DMT and MEG influences the reaction and the quality of the resulting polymer. Therefore, appropriate raw material preparation and process control are important.

8. Batch and Continuous Manufacturing Processes

PET produced through the DMT route can be manufactured by batch or continuous polymerisation.

Feature	Batch process	Continuous process
Production method	Separate batches	Continuous material flow
Polymer quality	May vary between batches	More uniform under normal operating conditions
Polymer handling	Chips are generally produced and handled	Can allow direct transfer to spinning
Flexibility	Greater process flexibility	Lower process flexibility
Production continuity	Batch-based	Continuous
Power failure sensitivity	Comparatively less sensitive	High sensitivity

The continuous process offers several advantages:

• More uniform polymer quality.
• Better control of polymer production.
• Improved fibre quality.
• Reduced oxidative degradation.
• Possibility of direct spinning without intermediate chip handling.

However, continuous production has limitations, including lower flexibility and greater sensitivity to power failure.

Main difference: The DMT route involves transesterification and methanol removal, whereas the PTA route involves direct esterification and water removal.

9. Advantages and Limitations of the DMT Route

Advantages

1. Formation of a chemically defined intermediate before polycondensation.
2. Better control of the starting material for polymerisation.
3. Fewer side reactions during subsequent polycondensation.
4. Better colour and thermal stability of the polyester melt, as described in the reference.
5. Good drawability of the resulting polymer.

Limitations

1. Methanol must be separated and recovered.
2. Transesterification requires a suitable catalyst.
3. The reaction requires controlled heating and material handling.
4. Unwanted side reactions can affect polymer quality.
5. The final molecular weight depends on efficient removal of MEG during polycondensation.

11. Frequently Asked Questions (FAQs)

1. What is the DMT route of polyester manufacturing?

The DMT route is a method of producing PET by reacting dimethyl terephthalate with monoethylene glycol through transesterification, followed by polycondensation.

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

The two main raw materials are dimethyl terephthalate (DMT) and monoethylene glycol (MEG).

3. What is transesterification in polyester manufacturing?

Transesterification is the reaction between DMT and MEG in which methyl ester groups are exchanged with glycol groups to form glycol terephthalate intermediates.

4. What is the by-product of transesterification?

Methanol is the main by-product of transesterification.

5. What is the intermediate formed in the DMT route?

Diglycol terephthalate (DGT) is identified as the intermediate in the uploaded textile manufacturing reference.

6. What is the function of a catalyst in the DMT route?

The catalyst increases the rate of the transesterification reaction between DMT and MEG.

7. What is the purpose of polycondensation?

Polycondensation increases the molecular weight of PET by joining smaller glycol terephthalate molecules into longer polymer chains.

8. Why is vacuum used during polycondensation?

Vacuum helps remove MEG produced during polymerisation, thereby promoting the formation of high-molecular-weight PET.

9. What is the difference between DMT and PTA routes?

The DMT route uses transesterification and produces methanol, whereas the PTA route uses direct esterification and produces water.

10. What is the final product of the DMT route?

The final product is high-molecular-weight polyethylene terephthalate (PET), which can be converted into chips or supplied directly to melt-spinning equipment.