Polyester fibre is produced mainly from polyethylene terephthalate (PET). In the TA or PTA route, PET is manufactured directly from terephthalic acid (TA/TPA) and monoethylene glycol (MEG). Unlike the DMT route, this process does not require an ester-interchange reaction with dimethyl terephthalate.

The first major reaction is direct esterification, in which terephthalic acid reacts with MEG and produces an ester intermediate with water as the main by-product. The resulting material then undergoes polycondensation under progressively reduced pressure to obtain high-molecular-weight PET suitable for fibre production.

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What Is the TA Route for Polyester?

The TA route, more commonly called the PTA route, is a method of manufacturing PET using:

• Purified terephthalic acid (PTA/TPA) as the acid component
• Monoethylene glycol (MEG) as the glycol component

PET is a step-growth condensation polymer. The two functional components react progressively, while small molecules are removed during the reaction.

The two industrial routes described in the supplied textile engineering reference are:

1. DMT route: DMT + MEG → ester interchange/transesterification
2. PTA route: TPA + MEG → direct esterification

After formation of the ester intermediate, the subsequent polycondensation stage is common to both routes.

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Why Was the TA/PTA Route Developed?

In the early development of PET technology, sufficiently pure polymer-grade terephthalic acid was not readily available. Therefore, the DMT route was developed because DMT could be purified effectively.

Later, processes for producing purified terephthalic acid were commercialized. The direct process using TPA consequently became increasingly important.

One source explains that crude TA contains impurities such as p-toluic acid and 4-carboxybenzaldehyde (4-CBA). Purification produces PTA suitable for direct use in PET polymerization.

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Raw Materials Used in the TA Route

The principal raw materials are:

1. Terephthalic acid

For PET production, high-purity terephthalic acid is required because impurities can affect polymer quality.

The supplied Springer reference gives the following example specifications for TPA:

Parameter	Source value
p-Carboxybenzaldehyde	≤25 ppm
p-Toluic acid	≤150 ppm
Total metal	≤9 ppm
Water	≤0.2 ppm
Ash	≤15 ppm
Particle size	50–600 μm

These values are source-specific specifications from the cited reference, not universal specifications for every modern PET plant.

2. Monoethylene glycol

MEG is the glycol component of PET.

The same reference gives:

• Boiling point: 195–198°C
• Density: 1.110–1.112 g cm⁻³ at 20°C
• Refractive index: 1.4330–1.4340 at 20°C
• Water content: 0.1%

Again, these are the values reported in the supplied reference.

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Principle of Polyester Manufacturing by TA Route

The TA route consists essentially of two major stages:

Stage 1: Direct esterification

TA/TPA reacts with MEG to form glycol terephthalate intermediates, with water being removed.

Stage 2: Polycondensation

The esterified material undergoes polycondensation. MEG is removed from the reaction system, and the molecular weight of PET increases.

The simplified overall sequence is:

Terephthalic acid + MEG
↓
Direct esterification
↓
Glycol terephthalate / oligomer
↓
Polycondensation
↓
PET polymer
↓
Melt spinning
↓
Polyester fibre

The reference describes DGT formation as an important first stage because it establishes the acid-to-glycol relationship before the high-molecular-weight polycondensation stage.

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Chemical Reaction in the TA Route

The direct esterification reaction can be represented as:

TPA + 2 MEG → DGT + 2 H₂O

where:

• TPA = terephthalic acid
• MEG = monoethylene glycol
• DGT = diglycol terephthalate
• H₂O = water

The reaction given in the reference shows TPA reacting with two molecules of MEG to form DGT and two molecules of water.

The next stage can be represented in simplified form as:

DGT → PET + MEG

The released MEG is removed from the reaction system, allowing the condensation reaction to proceed toward higher molecular weight.

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Flow Diagram of Polyester Manufacturing Using TA Route

Suggested process flow:

Purified TPA/PTA + MEG
↓
Mixing / Paste Preparation
↓
Direct Esterification Reactor
↓
Water Removal
↓
Low-Viscosity Esterified Product / Oligomers
↓
Thermal Stabilization / Catalyst Addition as Required
↓
Polycondensation Reactor
↓
Progressive Vacuum
↓
High-Molecular-Weight PET Melt
↓
Gear Pump / Discharge
↓
Direct Spinning or Pellet/Chip Production
↓
Polyester Filament or Staple Fibre

The supplied Springer reference gives essentially this sequence for the PTA route.

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Step 1: Preparation of TPA and MEG Mixture

One important difficulty in the PTA route is that terephthalic acid has low solubility in MEG.

According to the supplied reference, the TPA-MEG mixture is prepared at approximately:

• 240–260°C
• 400 kPa pressure
• TPA:MEG approximately 1:1 to 1:1.3

These are conditions reported for the process described in that reference and should not be treated as a universal operating recipe for every industrial plant.

The objective is to create a reaction mixture in which the acid and glycol can react effectively.

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Step 2: Direct Esterification

This is the characteristic first stage of the TA route.

Unlike the DMT process, there is no transesterification of a methyl ester. Instead, the carboxylic acid groups of TPA react directly with the hydroxyl groups of MEG.

Main reaction

TPA + 2MEG → DGT + 2H₂O

The water formed during esterification is continuously removed from the reaction system. Removal of water is important because the esterification reaction is reversible.

The source notes that, unlike the DMT route, a methanol reflux system is not required because the major by-product in direct esterification is water.

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Step 3: Removal of Water

Water is the principal by-product of direct esterification.

As the reaction proceeds, water is allowed to distil from the system.

This is one of the major process differences between the two PET routes:

DMT route	TA/PTA route
DMT + MEG	TPA + MEG
Ester interchange	Direct esterification
Methanol is produced	Water is produced
Methanol removal/reflux arrangement	Water removal by distillation
Esterification occurs indirectly through DMT	Acid reacts directly with MEG

The supplied references specifically note that no reflux attachment is required for methanol in the PTA route because water is the esterification by-product.

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Step 4: Formation of Esterified Oligomers

As esterification progresses, the system changes from a mixture of raw TPA and MEG into glycol terephthalate species and oligomers.

The purpose of this stage is not yet to produce the final high-molecular-weight PET.

Instead, the material is prepared for the subsequent polycondensation stage.

The intermediate formed in the first stage provides a more suitable starting material for molecular-weight development during polycondensation.

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Step 5: Addition of Stabilizer

After direct esterification, the low-viscosity melt may receive a thermal stabilizer before it is transferred to the polycondensation vessel.

The supplied reference specifically states that a thermal stabilizer can be added to the low-viscosity melt before transfer to the polycondensation vessel.

Phosphite or phosphate stabilizers are also described as being added after esterification to improve the time and temperature stability of the polymer melt.

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Step 6: Polycondensation of the Esterified Product

This is the stage where the molecular weight of PET increases substantially.

During polycondensation, hydroxyl-terminated and ester-containing molecules react to form longer PET chains.

A simplified representation is:

PET oligomers → higher-molecular-weight PET + MEG

The released MEG must be removed efficiently from the melt.

This is why vacuum becomes increasingly important as polycondensation proceeds.

The supplied reference explains that the attainment of final molecular weight is diffusion controlled and that progressive reduction of pressure helps obtain high molecular weight polymer.

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Why Is Vacuum Used During Polycondensation?

As PET molecular weight increases, the melt becomes increasingly viscous.

The volatile condensation product, mainly MEG, must escape from the polymer melt.

A progressively lower pressure helps remove the volatile material and drives the condensation process toward higher molecular weight.

In a continuous polycondensation system described in the source, the reactor operates under vacuum, with about 2 mm Hg reported as sufficient for a fibre-grade product with a number-average molecular weight of approximately 20,000 in that particular process description.

Important: This is a source-specific example, not a universal specification for all PET fibre plants.

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Catalysts Used in the TA Route

An important feature of the PTA route is that TPA itself accelerates the direct esterification reaction.

The Springer reference states that stronger acids or esters of titanic acid may be used when additional catalytic activity is required.

The TA route also differs from the DMT route because the direct esterification process does not require the same ester-interchange catalyst system used in the DMT process.

The absence of the metal catalyst used for ester interchange also changes the degradation behaviour during the later polycondensation stage.

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Side Reactions in the TA Route

The desired reaction is ester formation followed by polymer-chain growth. However, side reactions can occur.

One important side reaction is ether formation, which leads to the formation of diethylene glycol (DEG) units.

A simplified reaction is:

2 MEG → DEG + H₂O

The supplied reference states that ether/DEG formation is greater in the direct esterification process than in the ester-interchange route.

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Effect of DEG on PET

DEG becomes incorporated into the PET structure and changes its properties.

The reference reports that PET produced through the PTA route generally has:

• Lower average molecular weight
• Lower melting point
• Higher dye uptake

than PET produced through the DMT route, with these differences attributed in the reference to the higher DEG content of PTA-route PET.

Another source reports that DEG formation occurs during several stages of PET synthesis and can influence the structure and dye affinity of the resulting fibre.

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How Is DEG Formation Controlled?

The tendency toward ether formation can be reduced by adding a small amount of:

• NaOH
• An organic quaternary hydroxide

The supplied reference also states that using a higher temperature of 280–290°C can help suppress the tendency toward ether formation.

These conditions are source-specific process information and should not be interpreted as a universal operating window.

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Temperature and Pressure in the TA Route

The supplied sources describe several process conditions.

Direct esterification

One reference reports:

• 240–260°C
• Approximately 400 kPa
• TPA:MEG ratio of 1:1 to 1:1.3

Another source reports that direct esterification requires approximately:

• 250–260°C
• 1.5–2 bar overpressure

depending on reactor design and process arrangement.

Polycondensation

The polycondensation stage uses elevated temperature together with progressively reduced pressure.

A continuous process example reports a vacuum of approximately 2 mm Hg for production of a fibre-grade product.

Editorial Verification Required: Exact industrial temperatures, pressures, residence times and vacuum levels vary with plant design, throughput, catalyst system and product grade. The values above should therefore be presented as reported process examples, not as one universal PET recipe.

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Batch and Continuous TA-Route Processes

PET fibre-forming polymer can be manufactured by either:

• Batch processing
• Semicontinuous processing
• Continuous processing

The supplied textile engineering reference explains that continuous processing is favoured because it can provide more uniform fibre quality and avoids some of the handling and remelting operations associated with chip-based processing.

Batch process

Polymer is produced in batches and subsequently processed for spinning.

Semicontinuous process

Polymerization is continuous, but the polymer may still be converted to chips before spinning.

Continuous process

The polymer melt from the final polycondensation stage can be pumped directly to the spinning machines.

This is known as direct spinning.

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Direct Spinning of Polyester

When continuous polymerization is combined with direct spinning, the PET melt can be sent directly from the final polycondensation equipment to the spinning section.

This eliminates several intermediate operations:

PET melt → chip formation → chip handling → drying → remelting

Instead:

PET melt → spinning manifold → spinneret → fibre

The supplied reference describes this as the continuous process with direct spinning.

Another source explains that in melt-spinning plants, polymer can either be melted from chips in an extruder or pumped directly from the finisher of a continuous polycondensation plant, usually using gear pumps.

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Advantages of the TA Route

The TA/PTA route has several important advantages compared with the DMT route.

1. No DMT intermediate is required

The process starts directly with terephthalic acid and MEG.

2. No methanol is produced in the esterification stage

The main esterification by-product is water.

3. Direct esterification is self-catalysed to some extent

The source states that TPA itself accelerates the direct esterification step.

4. PTA can be less expensive than DMT

The supplied Springer reference identifies cost of TPA as one of the advantages of the PTA route.

5. Higher reaction product molecular weight at the esterification stage

The source states that the reaction product from the DE step has a higher molecular weight than that obtained from the ester-interchange step.

6. It became an important industrial route after PTA became economically available

The historical development of high-purity PTA made direct PET production practical on a large scale.

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TA Route vs DMT Route

Feature	TA/PTA Route	DMT Route
Main acid raw material	TPA/PTA	DMT
Glycol	MEG	MEG
First reaction	Direct esterification	Ester interchange/transesterification
Main by-product	Water	Methanol
DGT formation	Directly from TPA + MEG	Through ester interchange
Esterification catalyst requirement	TPA itself accelerates reaction; additional catalysts may be used	Ester-interchange catalysts required
Methanol reflux	Not required for the DE step	Required for methanol removal/reflux arrangement
Polycondensation	Common principle	Common principle
Industrial significance	Became increasingly important after PTA commercialization	Historically important

The underlying distinction between the two routes is clearly given in the supplied textile engineering reference.

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Problems and Process-Control Points in the TA Route

A good understanding of the TA route requires more than memorizing the flow chart.

1. Low solubility of TPA in MEG

TPA has low solubility in MEG, so appropriate temperature, pressure and mixing are required to establish the reaction mixture.

2. Water removal

Water must be removed during direct esterification so that the reaction can proceed effectively.

3. DEG formation

Ether formation can increase DEG content, which changes PET properties.

4. Thermal degradation

PET is exposed to high temperatures during polycondensation. The source notes that thermal degradation can occur because the PET decomposition temperature is close to the temperatures used in polycondensation.

5. Molecular-weight control

The final molecular weight depends strongly on effective removal of volatile condensation products under reduced pressure.

6. Raw-material purity

High-purity PTA is important because impurities such as 4-CBA and p-toluic acid can affect polymer quality.

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From PET Polymer to Polyester Fibre

Once the PET reaches the required molecular weight, the polymer can be sent to the spinning section.

For continuous direct spinning:

Final polycondensation reactor
↓
Polymer discharge
↓
Gear pump
↓
Spinning manifold
↓
Filter
↓
Spinneret
↓
Melt filaments
↓
Cooling/quenching
↓
Take-up
↓
Drawing / further processing

The subsequent fibre-processing route depends on whether the product is filament yarn, staple fibre, industrial yarn or another polyester product.

The supplied sources describe melt spinning and direct transfer of polymer melt to the spinning equipment.

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Why Is the TA Route Important in Textile Engineering?

For textile engineering students, the TA route is important because it connects polymer chemistry with fibre manufacturing.

A change at the polymerization stage can affect the final fibre.

For example:

Raw-material purity → affects polymer quality

DEG formation → affects polymer structure and dye affinity

Molecular weight → affects melt behaviour and fibre-forming capability

Thermal history → affects polymer degradation

Polycondensation conditions → affect the final polymer properties

Therefore, polyester fibre quality begins well before the polymer reaches the spinneret.

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Key Difference Between TA and DMT Routes

The easiest way to remember the difference is:

DMT route = ester interchange + methanol

TA/PTA route = direct esterification + water

After the initial stage, both routes proceed toward polycondensation and PET formation.

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Conclusion

Manufacturing of polyester using the TA or PTA route is based on the direct reaction of terephthalic acid with monoethylene glycol.

The process can be summarized as:

TPA/PTA + MEG
↓
Direct esterification
↓
Water removal
↓
Glycol terephthalate / oligomers
↓
Polycondensation under reduced pressure
↓
High-molecular-weight PET
↓
Melt spinning
↓
Polyester fibre

The major advantage of this route is that it avoids the DMT ester-interchange stage and produces water rather than methanol during the initial esterification. The subsequent polycondensation is essentially the common molecular-weight-building stage of PET manufacture.

For examination purposes, remember the three most important points:

TA/PTA + MEG → direct esterification → water

Polycondensation → MEG removal + molecular-weight increase

PET melt → spinning → polyester fibre

5. QUICK REVISION

Manufacturing of Polyester Using TA Route — Short Notes

1. The TA route is also called the PTA route or direct esterification route.
2. The main raw materials are terephthalic acid (TPA/PTA) and monoethylene glycol (MEG).
3. PET is a step-growth condensation polymer.
4. The first stage is direct esterification.
5. The main reaction is: TPA + 2MEG → DGT + 2H₂O
6. Water is the principal by-product of direct esterification.
7. TPA has low solubility in MEG, so elevated temperature and pressure are used in the reported process.
8. The esterified product is transferred to the polycondensation stage.
9. Polycondensation increases the molecular weight of PET.
10. Vacuum helps remove volatile MEG and promotes molecular-weight development.
11. DEG can form through etherification of MEG.
12. Higher DEG content can influence PET melting point and dye affinity.
13. The PTA route does not require the DMT ester-interchange step.
14. The route can be operated as batch, semicontinuous or continuous processing.
15. Continuous polymerization can be combined with direct spinning.

One-line exam answer

In the TA/PTA route, polyester is manufactured by direct esterification of terephthalic acid with monoethylene glycol, followed by polycondensation under reduced pressure to produce high-molecular-weight PET for fibre production.

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6. FAQS

1. What is the TA route in polyester manufacturing?

The TA route is the process of manufacturing PET using terephthalic acid (TA/TPA/PTA) and monoethylene glycol (MEG). The first stage is direct esterification, followed by polycondensation.

2. What is the main reaction in the TA route?

The simplified first-stage reaction is:

TPA + 2MEG → DGT + 2H₂O

DGT then undergoes polycondensation to form PET.

3. What is the main by-product in the TA route?

Water is the main by-product of the direct esterification stage. This is different from the DMT route, where methanol is produced during ester interchange.

4. Why is PTA used instead of DMT?

The development of economical, high-purity PTA made it possible to react TPA directly with MEG, eliminating the DMT ester-interchange stage.

5. What is direct esterification?

Direct esterification is the reaction of the carboxylic acid groups of TPA with the hydroxyl groups of MEG to form ester groups, with water being produced.

6. Why is vacuum applied during PET polycondensation?

Vacuum helps remove volatile condensation products, particularly MEG, from the polymer melt. Progressive pressure reduction supports the development of high molecular weight.

7. What is DEG in polyester?

DEG means diethylene glycol. It can form through etherification of MEG during PET manufacture and can become incorporated into the polymer structure.

8. How does DEG affect polyester?

The supplied reference associates higher DEG content with changes including lower melting point and higher dye uptake in PTA-route PET.

9. Is the polycondensation stage different for DMT and TA routes?

The basic polycondensation stage is common to both routes. The major difference occurs in the first stage: DMT uses ester interchange, whereas TA uses direct esterification.

10. Can the TA route be connected directly to spinning?

Yes. In a continuous process, PET melt from the final polycondensation stage can be pumped directly to the spinning machines, avoiding intermediate chip production, handling, drying and remelting.

Source basis

This article is grounded primarily in the uploaded references Manufactured Fibre Technology (Springer, 1997), Synthetic Fibers by Franz Fourne, and Synthetic Fibres: Nylon, Polyester, Acrylic, Polyolefin. The process conditions and numerical values are retained as reported in those sources rather than presented as universal modern plant specifications.