Properties of Polyester Fibre: Physical, Mechanical, Thermal and Chemical Properties
Introduction
Polyester is an important synthetic fibre used in the textile industry. It is widely valued for its strength, elastic recovery, low moisture regain, chemical resistance and biological resistance.
The most commonly used textile polyester is polyethylene terephthalate (PET). Its properties are influenced by its molecular structure, crystallinity, orientation and manufacturing conditions.
The properties of polyester fibre can be classified into the following groups:
- Physical properties
- Mechanical properties
- Thermal properties
- Shrinkage properties
- Chemical properties
- Dyeing properties
- Resistance to sunlight
- Biological resistance
1. Physical Properties of Polyester Fibre
Physical properties describe the appearance, density and moisture behaviour of polyester fibre.
| Property | Value |
|---|---|
| Appearance | Smooth surface |
| Density | 1.38 g/cm³ |
| Moisture regain at 65% RH | 0.4% |
| Moisture regain at 100% RH | 0.6–0.8% |
1.1 Appearance
Polyester fibre has a smooth surface. Its appearance can vary according to the fibre cross-section, lustre and manufacturing modifications.
1.2 Density
The density of polyester fibre is 1.38 g/cm³.
Density is the mass of a material per unit volume. It is an important physical property when comparing different textile fibres.
1.3 Moisture Regain
Polyester is a low-moisture-regain fibre. Its moisture regain is:
- 0.4% at 65% relative humidity.
- 0.6–0.8% at 100% relative humidity.
Moisture regain is the amount of moisture absorbed by a fibre from the surrounding atmosphere, expressed as a percentage of its dry weight.
The low moisture regain of polyester means that it absorbs very little moisture from the atmosphere.
2. Mechanical Properties of Polyester Fibre
Mechanical properties describe the behaviour of polyester fibre when it is subjected to forces such as tension and extension.
The important mechanical properties are tenacity, elongation and elastic recovery.
2.1 Tenacity
Tenacity is the tensile force required to break a fibre relative to its linear density.
The tenacity value is given as:
- 3–5 spg
- 6–9 gpd
These are the two values shown in the property table.
2.2 Elongation
Elongation is the increase in the length of a fibre when a tensile force is applied. It is expressed as a percentage of the original length.
The elongation of polyester fibre is given as:
- 25–35%
- 20%
The values are retained as listed in the property table.
2.3 Elastic Recovery
Elastic recovery is the ability of a fibre to return towards its original length after the removal of an applied force.
The elastic recovery of polyester is:
- 96% at 2% extension
- 90% at 5% extension
This indicates good elastic recovery under the stated extension conditions.
2.4 Summary of Mechanical Properties
| Property | Value |
|---|---|
| Tenacity | 3–5 spg (6–9 gpd) |
| Elongation | 25–35% (20%) |
| Elastic recovery at 2% extension | 96% |
| Elastic recovery at 5% extension | 90% |
3. Thermal Properties of Polyester Fibre
Thermal properties describe the behaviour of polyester fibre when it is exposed to heat.
The important thermal properties are shown below.
| Thermal property | Value |
|---|---|
| Tg | 78°C |
| Tsi | 135°C |
| Tstick | 150°C |
| Loss in St. | 240–248°C |
| Tm | 260°C |
3.1 Glass Transition Temperature (Tg)
The glass transition temperature (Tg) is the temperature range in which the amorphous regions of a polymer change from a hard, glassy condition towards a more flexible condition.
The Tg value given for polyester is 78°C.
3.2 Tsi (Safe Ironing Temperature)
The property table gives the Tsi value as 135°C.
The abbreviation is retained as listed.
3.3 Tstick (Sticking Temperature)
The property table gives the Tstick value as 150°C.
The abbreviation is retained as listed.
3.4 Loss in Strength
The property table gives the value for loss in strength as 240–248°C.
This entry indicates a temperature-related strength property of polyester fibre.
3.5 Melting Temperature (Tm)
The melting temperature (Tm) is the temperature at which the crystalline regions of a polymer melt.
The melting temperature given for polyester is 260°C.
4. Shrinkage Properties of Polyester Fibre
Shrinkage is the reduction in the length of a fibre when it is exposed to heat or moisture.
The shrinkage values of polyester fibre are:
| Treatment condition | Shrinkage |
|---|---|
| Boiling water | 8–9% |
| Hot air at 180°C for 30 minutes | 5–8% |
4.1 Shrinkage in Boiling Water
The shrinkage of polyester fibre in boiling water is 8–9%.
4.2 Shrinkage in Hot Air
When polyester fibre is exposed to hot air at 180°C for 30 minutes, its shrinkage is 5–8%.
Shrinkage behaviour is important during textile processing because dimensional changes can affect the dimensions and appearance of yarns and fabrics.
5. Chemical Properties of Polyester Fibre
Chemical properties describe the resistance of polyester fibre to different chemicals.
Polyester shows resistance to several acids, weak alkalis, oxidising agents, reducing agents, solvents and cleaning agents. However, certain chemical treatments can damage or swell the fibre.
5.1 Resistance to Acids
Polyester shows high resistance to organic and mineral acids.
| Chemical | Action on polyester |
|---|---|
| Organic and mineral acids | High resistance |
| Weak acids | No harm |
| Strong acids | Do not attack |
5.2 Resistance to Alkalis
Polyester shows resistance to weak alkaline solutions. However, hot alkaline solutions can hydrolyse the fibre surface.
| Chemical | Action on polyester |
|---|---|
| Weak alkalis | No harm |
| Hot alkaline solutions | Hydrolyse the surface |
Hydrolysis is the chemical breakdown of a material through reaction with water. In polyester, alkaline conditions can cause damage to the polymer structure.
5.3 Resistance to Oxidising and Reducing Agents
Polyester is reported to be extremely stable against oxidising and reducing agents.
This resistance is useful when polyester is exposed to different chemical treatments during textile processing.
5.4 Action of Benzoic Acid and m-Cresol
Benzoic acid and m-cresol cause approximately 2% fibre swelling.
Fibre swelling means an increase in the dimensions of the fibre due to the action of a chemical.
5.5 Resistance to Solvents and Cleaning Agents
Polyester shows resistance to several solvents and cleaning agents.
The reported behaviour is:
| Chemical | Action on polyester |
|---|---|
| Alcohols | No chemical action |
| Ketones | No chemical action |
| Soaps | No chemical action |
| Detergents | No chemical action |
| Dry-cleaning agents | No chemical action |
5.6 Summary of Chemical Resistance
| Sr. No. | Chemical | Action |
|---|---|---|
| 1 | Organic and mineral acids | High resistance |
| 2 | Weak acids | No harm |
| 3 | Strong acids | Do not attack |
| 4 | Hot alkaline solutions | Hydrolyse the surface |
| 5 | Weak alkalis | No harm |
| 6 | Oxidising and reducing agents | Extremely stable |
| 7 | Benzoic acid and m-cresol | Fibre swelling (2%) |
| 8 | Alcohols, ketones, soaps, detergents and dry-cleaning agents | No chemical action |
6. Dyeing Properties of Polyester Fibre
Polyester is a hydrophobic synthetic fibre and is difficult to dye using many conventional dye classes.
Disperse dyes are used for dyeing normal polyester fibre. The supplied property table identifies the high-temperature high-pressure (HTHP) method for polyester dyeing.
6.1 Disperse Dyeing
Disperse dyes are suitable for polyester because they can diffuse into the fibre under appropriate dyeing conditions.
Normal polyester is described as dyeable with disperse dyes.
6.2 Modified Polyester
Modified polyester varieties can have different dyeing characteristics.
These include:
- Polyester modified to improve dyeability with disperse dyes.
- Cationic-dyeable polyester produced by introducing an anionic sulphonate group into the polymer structure.
Therefore, the dyeing behaviour of modified polyester can differ from that of normal polyester.
7. Effect of Sunlight on Polyester Fibre
The supplied property data indicate that polyester fibre becomes yellow when exposed to sunlight.
The reported strength retention is 70–80%.
| Property | Reported behaviour |
|---|---|
| Effect of sunlight | Yellowing |
| Strength retention | 70–80% |
The exposure duration and test conditions associated with this strength-retention value are not specified.
8. Biological Resistance of Polyester Fibre
Polyester has excellent biological resistance.
It shows resistance to biological agents such as:
- Mildew
- Fungi
- Bacteria
This property helps polyester resist biological deterioration.
9. Advantages of Polyester Fibre Based on Its Properties
The important advantages of polyester fibre include:
- Good mechanical properties: Polyester has useful tenacity and elongation characteristics.
- Good elastic recovery: It shows 96% recovery at 2% extension and 90% recovery at 5% extension.
- Low moisture regain: Its moisture regain is 0.4% at 65% RH.
- Chemical resistance: It resists several acids, weak alkalis, oxidising and reducing agents, and cleaning agents.
- Biological resistance: It has excellent resistance to mildew, fungi and bacteria.
- Useful thermal properties: Its thermal behaviour is important during heat treatment and textile processing.
- Disperse dyeability: Normal polyester can be dyed using disperse dyes.
10. Complete Property Chart of Polyester Fibre
| Sr. No. | Property | Value |
|---|---|---|
| 1 | Appearance | Smooth surface |
| 2 | Density | 1.38 g/cm³ |
| 3 | Tenacity | 3–5 spg (6–9 gpd) |
| 4 | Elongation | 25–35% (20%) |
| 5 | Elastic recovery | 96% at 2% extension; 90% at 5% extension |
| 6 | Moisture regain | 0.4% at 65% RH; 0.6–0.8% at 100% RH |
| 7 | Tg | 78°C |
| 8 | Tsi | 135°C |
| 9 | Tstick | 150°C |
| 10 | Loss in St. | 240–248°C |
| 11 | Tm | 260°C |
| 12 | Shrinkage in boiling water | 8–9% |
| 13 | Shrinkage in hot air | 5–8% at 180°C for 30 minutes |
| 14 | Effect of sunlight | Yellowing |
| 15 | Strength retention after sunlight exposure | 70–80% |
| 16 | Biological resistance | Excellent |
11. Frequently Asked Questions (FAQs)
1. What is the density of polyester fibre?
The density of polyester fibre is 1.38 g/cm³.
2. What is the moisture regain of polyester fibre?
The moisture regain is 0.4% at 65% relative humidity and 0.6–0.8% at 100% relative humidity.
3. What is the tenacity of polyester fibre?
The tenacity is given as 3–5 spg (6–9 gpd).
4. What is the elongation of polyester fibre?
The elongation is given as 25–35% (20%).
5. What is the elastic recovery of polyester fibre?
The elastic recovery is 96% at 2% extension and 90% at 5% extension.
6. What is the Tg of polyester fibre?
The Tg value is 78°C.
7. What is the melting temperature of polyester fibre?
The melting temperature (Tm) is 260°C.
8. Which dyes are used for polyester fibre?
Normal polyester fibre is dyed using disperse dyes, including by the high-temperature high-pressure dyeing method.
9. What is the shrinkage of polyester fibre in boiling water?
The shrinkage in boiling water is 8–9%.
10. Is polyester resistant to biological attack?
Yes. Polyester has excellent biological resistance and resists mildew, fungi and bacteria.
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