Chemical Properties of PET

Chemical Properties of Polyethylene Terephthalate (PET) Based on Its Molecular Structure

Introduction

Polyethylene Terephthalate (PET) is a widely used thermoplastic polymer known for its excellent balance of properties. These characteristics are directly attributable to its unique molecular structure, which comprises repeating units of terephthalate and ethylene glycol.

Molecular Structure Overview

The repeating unit of PET consists of a terephthalate moiety (a benzene ring with two carboxyl groups) linked by ester bonds to an ethylene glycol moiety (-O-CH2-CH2-O-). The presence of the rigid benzene ring, the flexible ethylene chain, and the polar ester linkages all contribute significantly to PET's overall chemical and physical behavior.

Key Chemical Properties and Structural Influences

1. Chemical Resistance

PET exhibits good to excellent resistance to a variety of chemicals, which is crucial for its applications, particularly in packaging and textiles .

•Excellent resistance to alcohols, aliphatic hydrocarbons, oils, grease, and diluted acids: The relatively stable ester linkages and the hydrophobic nature of the benzene ring and ethylene chain contribute to this resistance. The ester bonds are generally stable against these types of chemicals under normal conditions.

•Moderate resistance to diluted alkalis, aromatic & halogenated hydrocarbons: While generally resistant, strong alkalis can cause saponification (hydrolysis of the ester bonds), leading to degradation of the polymer. Aromatic and halogenated hydrocarbons can cause swelling or dissolution due to similar polarity or solvent effects.

2. Hydrolytic Stability

PET's ester linkages are susceptible to hydrolysis, especially in the presence of water at elevated temperatures or in acidic/alkaline environments. This is a significant limitation for PET, as moisture can lead to chain scission and a reduction in molecular weight, impacting mechanical properties .

3. Thermal Stability

The presence of the rigid benzene rings in the polymer backbone contributes to PET's relatively high glass transition temperature (Tg) (65-80°C) and melting temperature (Tm) (240-270°C) . These high thermal properties are due to the restricted rotation around the bonds within the benzene ring and the strong intermolecular forces (dipole-dipole interactions from ester groups and van der Waals forces) between adjacent polymer chains. This allows PET to maintain its structural integrity over a broad range of temperatures.

4. Crystallinity

PET is a semi-crystalline polymer, typically achieving 40-50% crystallinity . The ability to crystallize is influenced by the regularity of its molecular structure. The linear arrangement of the repeating units allows for efficient packing of polymer chains, forming crystalline regions. These crystalline regions contribute to PET's high strength, stiffness, and barrier properties. The ethylene glycol segment provides some flexibility, allowing the chains to align, while the rigid terephthalate units promote ordered structures.

5. Inertness and Safety

PET is largely considered an inert polymer, especially in its solid state. Its non-reactive quality makes it safe for contact with foods and beverages, as approved by regulatory bodies like the FDA . This inertness stems from the stable covalent bonds within its structure and the absence of highly reactive functional groups that could easily leach into contents.

Conclusion

The molecular architecture of PET, characterized by its alternating rigid aromatic rings and flexible aliphatic chains connected by ester linkages, dictates its impressive array of chemical properties. These structural features confer excellent chemical resistance to many substances, good thermal stability, and the ability to form semi-crystalline structures, all of which are critical for its widespread industrial applications.

References

[1] [Polyethylene Terephthalate (PET) - Uses, properties & structure](https://www.specialchem.com/plastics/guide/polyethylene-terephthalate-pet-plastic ) - SpecialChem