Chemical Properties of Polyester

Polyester, particularly poly(ethylene terephthalate) (PET), is a condensation polymer containing ester linkages in its main chain. These ester groups determine much of its chemical behaviour. Polyester fibres generally show good resistance to many acids, alkalis, oxidising agents, reducing agents, solvents, microorganisms and sunlight, but the ester bonds can undergo hydrolysis and alkaline saponification under sufficiently severe conditions.

1. Resistance to Acids

Polyester has good resistance to weak inorganic acids. It also has good resistance to strong inorganic acids under many textile-use conditions, although prolonged or severe acid treatment can cause degradation.

The ester linkage is susceptible to acid-catalysed hydrolysis. Strong acids can therefore attack the polyester chain under sufficiently severe conditions. For example, concentrated sulphuric acid can depolymerize PET and produce terephthalic acid.

In textile processing: polyester can tolerate many ordinary acidic treatments, but concentrated acids and severe temperature–time conditions should be avoided.

2. Resistance to Alkalis

Polyester shows good resistance to dilute alkalis, but concentrated alkalis, particularly at elevated temperature, can attack the fibre.

The characteristic reaction is alkaline hydrolysis or saponification of the ester groups. Caustic soda cleaves the ester groups progressively from the fibre surface. This property is deliberately used in alkaline weight reduction (caustic treatment) of polyester fibres to produce finer, silk-like effects.

The rate of saponification is affected by the physical state of the fibre. Drawing and heat-setting reduce the rate of saponification.

3. Hydrolysis

Hydrolysis is an important chemical characteristic of PET because its polymer chain contains ester bonds.

Water can hydrolyse polyester under sufficiently severe conditions of temperature and pressure. Industrial studies on PET waste report hydrolysis with water at 150–250 °C under pressure, producing terephthalic acid and ethylene glycol.

This is also why PET chips must be thoroughly dried before melt processing. Moisture can cause hydrolysis of ester groups during melting, reducing polymer quality.

4. Resistance to Oxidising Agents

Polyester has good resistance to oxidising agents compared with several other textile fibres. The comparative chemical-resistance table in Synthetic Fibres: Nylon, Polyester, Acrylic, Polyolefin classifies polyester as resistant to oxidising agents.

This resistance is useful because polyester can withstand many commonly encountered bleaching and textile-processing environments.

5. Resistance to Reducing Agents

Polyester also shows good resistance to reducing agents under ordinary conditions. However, the exact behaviour depends on the particular chemical, concentration, temperature and exposure time.

Therefore, chemical resistance should not be interpreted as complete resistance to every reducing chemical.

6. Resistance to Organic Solvents

PET is generally resistant to many commonly used solvents, but it is not completely inert to solvents.

The source material lists several chemicals capable of dissolving or strongly affecting polyester, including phenol, cresols, certain chlorinated aromatic solvents and DMF under appropriate conditions.

This solvent behaviour is related to the chemical structure and relatively strong intermolecular interactions of PET.

7. Resistance to Microorganisms

Polyester has good resistance to biological attack. Unlike natural fibres such as cotton and wool, it does not readily provide a suitable substrate for ordinary microbial degradation.

The comparative chemical-properties table reports polyester as resistant to biological agents such as mildew, fungus and bacteria.

This contributes to the durability of polyester fabrics during normal use.

8. Resistance to Sunlight and Weathering

Polyester has good stability in sunlight, although the degree of stability can depend on fibre formulation, dyes, additives and exposure conditions. The comparative table classifies polyester’s sunlight stability as good.

UV stabilisers and other modifications can be used where greater resistance is required.

9. Effect of Chemical Structure on Chemical Properties

The chemical behaviour of polyester is closely connected with its molecular structure. PET contains repeating ester groups:

–CO–O–

These groups are relatively stable under ordinary textile-use conditions, which gives polyester its useful chemical resistance. However, because ester linkages can be cleaved by hydrolysis, polyester is not completely resistant to strong acids, concentrated alkalis or severe hydrothermal treatment.

This explains an important practical point:

Polyester is chemically resistant in normal textile conditions, but its ester bonds can be chemically cleaved under sufficiently severe conditions.

Chemical Resistance of Polyester at a Glance

Chemical environment	Behaviour of polyester
Weak inorganic acids	Resistant
Strong inorganic acids	Generally resistant, but can degrade under severe conditions
Dilute alkalis	Resistant
Concentrated alkalis	Degrades, especially at high temperature
Oxidising agents	Resistant
Reducing agents	Resistant under ordinary conditions
Water	Resistant under normal conditions; hydrolysis occurs under severe conditions
Microorganisms	Resistant
Sunlight	Good stability
Strong chemical hydrolysis	Ester bonds can be cleaved

The comparative chemical-resistance data in the source specifically classify polyester as resistant to weak inorganic acids, strong inorganic acids with degradation possible, dilute alkalis, oxidising agents and reducing agents, while concentrated alkalis cause degradation at elevated conditions.

Key Point for Textile Engineering Students

The most important chemical feature to remember is the ester linkage in PET. It gives polyester good resistance to many chemicals, but it also makes the polymer susceptible to hydrolysis and alkaline saponification under severe conditions. This relationship between molecular structure and chemical behaviour is important in dyeing, finishing, caustic weight reduction, recycling and polyester processing.