Polyester is one of the most important synthetic fibres used in the textile industry. It is widely used for apparel, home textiles, industrial textiles, and blended fabrics because of its useful combination of strength, elastic recovery, low moisture regain, thermoplastic behaviour, dimensional stability, and resistance to biological attack.

For textile engineering students, understanding the physical properties of polyester fibre is important because these properties influence fibre processing as well as the performance of the final textile product.

The following table presents the important physical properties of polyester based on the supplied reference data.

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Physical Properties of Polyester Fibre: Complete Table

Sr. No.	Property	Value
1	Appearance	Smooth surface
2	Density	1.38 g/cm³
3	Tenacity	3–5 gpd (6–9 gpd)
4	Elongation	25 to 35% (20%)
5	Elastic Recovery	96% at 2% extension (90% @ 5%)
6	Moisture Regain	0.4% at 65% RH; 0.6–0.8% at 100% RH
7	Thermal Properties	Tg: 78°C; Safe ironing temperature (Tsi): 135°C; Tstick: 150°C; Loss in strength: 240–248°C; Tm: 260°C
8	Shrinkage	Boiling water: 8 to 9%; Hot air (180°C, 30 min): 5 to 8%
9	Effect of Sunlight	Yellow (70 to 80% strength retention)
10	Biological Resistance	Excellent

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1. Appearance of Polyester Fibre

The supplied table describes polyester fibre as having a:

Smooth surface

The surface appearance of a polyester fibre depends partly on the fibre manufacturing process and cross-sectional shape. Polyester can also be manufactured in modified cross-sectional forms for specific applications.

A smooth fibre surface influences characteristics such as:

• Fibre friction
• Handle
• Lustre
• Fibre-to-fibre contact
• Fabric appearance

Key point

Appearance of polyester = Smooth surface

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2. Density of Polyester

The density of polyester given in the supplied table is:

1.38 g/cm³

Density is the mass of fibre material per unit volume.

It is an important property in textile calculations because fibre density influences the relationship between mass, volume, and linear density.

For a given fibre mass, a higher-density material occupies less volume than a lower-density material.

Key point

Density of polyester = 1.38 g/cm³

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3. Tenacity of Polyester

Tenacity is the breaking force of a fibre expressed in relation to its linear density.

The supplied table gives:

3–5 gpd (6–9 gpd)

Here, gpd means grams per denier.

The value in parentheses has been retained as given in the supplied table.

Tenacity is an important property because it indicates the ability of the fibre to resist tensile forces before breaking.

Importance of tenacity

Good tensile strength is useful during:

• Spinning
• Drawing
• Weaving
• Knitting
• Sewing
• Industrial textile applications

The actual tenacity of a polyester fibre can vary according to fibre type, molecular orientation, drawing, and processing conditions.

Key point

Tenacity of polyester = 3–5 gpd (6–9 gpd)

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4. Elongation of Polyester

Elongation is the increase in length of a fibre when it is subjected to tensile force, normally expressed as a percentage of its original length.

The supplied table gives:

25 to 35% (20%)

Elongation is an important mechanical property because it indicates how much the fibre can extend before breaking.

Polyester fibre properties can be modified by drawing. Drawing increases molecular orientation and changes the balance between strength, elongation, and modulus.

Key point

Elongation of polyester = 25 to 35% (20%)

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5. Elastic Recovery of Polyester

Elastic recovery indicates the ability of a fibre to recover its original length after being extended and released.

The supplied table gives:

96% at 2% extension (90% @ 5%)

Good elastic recovery is one reason polyester fabrics generally show good resistance to deformation.

It contributes to:

• Shape retention
• Dimensional stability
• Crease resistance
• Recovery from small extensions
• Good appearance retention

For example, when a polyester fabric is stretched during use, good recovery helps it return toward its original dimensions after the applied force is removed.

Key point

Elastic recovery = 96% at 2% extension and 90% at 5% extension

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6. Moisture Regain of Polyester

Polyester has a low moisture regain.

The supplied table gives:

• 0.4% at 65% RH
• 0.6–0.8% at 100% RH

Here, RH means relative humidity.

The low moisture regain of polyester is an important characteristic of the fibre.

Effects of low moisture regain

It contributes to:

• Quick drying
• Low moisture absorption
• Good dimensional stability
• Poor moisture absorption compared with hydrophilic fibres
• Greater tendency to accumulate static electricity

The moisture regain increases as the relative humidity increases, as shown by the values in the supplied table.

Key point

Moisture regain = 0.4% at 65% RH and 0.6–0.8% at 100% RH

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7. Thermal Properties of Polyester

Polyester is a thermoplastic fibre. Therefore, temperature has a major influence on its physical behaviour and textile processing.

The supplied table gives the following thermal properties:

Thermal Property	Value
Glass transition temperature (Tg)	78°C
Safe ironing temperature (Tsi)	135°C
Tstick	150°C
Loss in strength	240–248°C
Melting temperature (Tm)	260°C

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Glass Transition Temperature (Tg)

The supplied table gives:

Tg = 78°C

The glass transition temperature is associated with a significant change in molecular mobility in the amorphous regions of the polymer.

Tg is important in textile processing because temperature around this region can affect:

• Molecular mobility
• Drawing behaviour
• Heat-setting
• Dimensional stability
• Structural relaxation

Key point

Tg of polyester = 78°C

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Safe Ironing Temperature (Tsi)

Tsi means safe ironing temperature.

The supplied table gives:

Tsi = 135°C

This is an important practical thermal property for textile use because ironing exposes the fabric to elevated temperatures.

Keeping the ironing temperature within an appropriate range helps reduce the risk of thermal damage to the polyester fabric.

Key point

Safe ironing temperature of polyester = 135°C

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Tstick

The supplied table gives:

Tstick = 150°C

This temperature is listed separately from the safe ironing temperature in the supplied property table.

Key point

Tstick = 150°C

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Loss in Strength

The supplied table gives:

240–248°C

as the temperature range associated with loss in strength.

This demonstrates why controlling temperature is important during high-temperature processing of polyester.

Excessive temperature can adversely affect the fibre’s mechanical properties.

Key point

Loss in strength = 240–248°C

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Melting Temperature (Tm)

The supplied table gives:

Tm = 260°C

Polyester is thermoplastic, so it softens and eventually melts when sufficiently heated.

This property is fundamental to the manufacture of polyester because PET is produced and processed through melt-based technology, including melt spinning.

Key point

Melting temperature of polyester = 260°C

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8. Shrinkage of Polyester

Shrinkage is an important physical property of polyester because the fibre can undergo dimensional changes when exposed to heat or hot water.

The supplied table gives:

Treatment condition	Shrinkage
Boiling water	8 to 9%
Hot air at 180°C for 30 min	5 to 8%

These values show that polyester shrinkage depends on the treatment conditions.

Why does polyester shrink?

During fibre production, molecular chains become oriented, particularly during drawing. Heating can allow some of this oriented structure to relax, resulting in dimensional contraction.

This is why heat-setting is important in polyester processing.

Importance of shrinkage control

Shrinkage affects:

• Fabric dimensions
• Garment dimensions
• Seam stability
• Fabric appearance
• Dimensional stability
• Processing consistency

Key points

Shrinkage in boiling water = 8–9%

Shrinkage in hot air at 180°C for 30 min = 5–8%

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9. Effect of Sunlight on Polyester

The supplied table gives the effect of sunlight as:

Yellow (70 to 80% strength retention)

Thus, according to the supplied data, exposure to sunlight is associated with yellowing and the reported strength retention is 70–80%.

Sunlight contains ultraviolet radiation that can cause photochemical changes in polymeric materials.

The effect of sunlight can depend on:

• Exposure duration
• UV intensity
• Fibre structure
• Stabilizers
• Pigments
• Dyes
• Environmental conditions

The value in the supplied table should therefore be understood as the reported property value for the specified reference condition.

Key point

Effect of sunlight = Yellow; 70–80% strength retention

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10. Biological Resistance of Polyester

The supplied table rates the biological resistance of polyester as:

Excellent

Polyester generally has good resistance to biological attack compared with fibres that are more susceptible to microbial degradation.

This property is useful in applications where resistance to biological agents is required.

Importance of biological resistance

Good biological resistance can contribute to:

• Longer service life
• Better storage stability
• Resistance to microbial attack
• Suitability for industrial textile applications

Key point

Biological resistance of polyester = Excellent

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Physical Properties of Polyester: Summary Table

For quick reference, the complete property table is given below.

Property	Value
Appearance	Smooth surface
Density	1.38 g/cm³
Tenacity	3–5 gpd (6–9 gpd)
Elongation	25 to 35% (20%)
Elastic recovery	96% at 2% extension (90% @ 5%)
Moisture regain	0.4% at 65% RH; 0.6–0.8% at 100% RH
Glass transition temperature (Tg)	78°C
Safe ironing temperature (Tsi)	135°C
Tstick	150°C
Loss in strength	240–248°C
Melting temperature (Tm)	260°C
Shrinkage in boiling water	8–9%
Shrinkage in hot air at 180°C for 30 min	5–8%
Effect of sunlight	Yellow; 70–80% strength retention
Biological resistance	Excellent

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Factors Affecting the Physical Properties of Polyester

The physical properties of polyester are influenced by the structure of the fibre and the conditions used during its manufacture and processing.

1. Molecular Weight

Molecular weight influences the behaviour of PET during polymer processing and fibre formation.

2. Molecular Orientation

During drawing, polymer chains become oriented in the fibre direction. This has an important effect on:

• Tenacity
• Elongation
• Modulus
• Recovery

3. Crystallinity

Polyester contains crystalline and amorphous regions. Changes in crystallinity can affect:

• Density
• Mechanical properties
• Thermal behaviour
• Dimensional stability

4. Drawing

Drawing is one of the most important processes for developing the required mechanical properties of polyester fibre.

It changes molecular orientation and consequently affects the relationship between strength and elongation.

5. Heat Treatment

Heat treatment can cause molecular relaxation and structural changes.

It is particularly important for controlling:

• Shrinkage
• Dimensional stability
• Crystallinity
• Fibre structure
• Thermal behaviour

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Importance of Polyester Physical Properties in Textile Processing

The physical properties of polyester determine how the fibre behaves during different textile operations.

Property	Importance in textiles
Smooth surface	Influences fibre handle and appearance
Density	Important for mass-volume calculations
Tenacity	Determines resistance to tensile forces
Elongation	Determines extension before break
Elastic recovery	Helps maintain shape and dimensions
Low moisture regain	Gives quick drying but may increase static
Tg	Important for thermal and structural changes
Safe ironing temperature	Important for garment care
Melting temperature	Important for melt processing and spinning
Shrinkage	Important for dimensional stability
Sunlight resistance	Important for outdoor exposure
Biological resistance	Important for durability and storage

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Physical Properties and Fibre Structure

A useful concept for textile engineering students is that polyester’s physical properties are closely connected with its internal structure.

The relationship can be represented as:

Polymer structure
↓
Spinning conditions
↓
Molecular orientation + crystallinity
↓
Fibre structure
↓
Physical properties

For example:

Drawing → orientation changes → strength and elongation change

Heat treatment → molecular relaxation/crystallization → shrinkage and dimensional stability change

Therefore, the properties of polyester are not determined only by its chemical composition. Manufacturing and processing conditions also play an important role.