PET's Degradation by Acids/Bases at High Temp

PET, or polyethylene terephthalate, can be used in many places because of its good performance. However, if it is placed at high temperature and encounters strong acids and alkalis, its performance will be greatly reduced.

1. Characteristics and potential risks of PET

In industrial production and daily life, PET (polyethylene terephthalate) is widely used in food packaging, fiber textiles, electronic appliances and other fields due to its good mechanical properties, transparency and chemical stability. However, when PET is in a high temperature environment and encounters strong acids and alkalis, its stable chemical structure will be severely challenged and its performance will change significantly. ​

2. Mechanism of corrosion of PET by high temperature and strong acid

From the chemical structure point of view, the PET molecular chain contains a large number of ester groups (-COO-), which makes it very easy to undergo hydrolysis reaction in a high temperature and strong acid and alkali environment.

When PET is exposed to high temperature and strong acids, such as concentrated sulfuric acid and concentrated nitric acid, hydrogen ions (H⁺) will accelerate the attack on the ester group and cause the ester bond to break. Take concentrated sulfuric acid as an example.

Under high temperature conditions, concentrated sulfuric acid is not only highly acidic, but also exhibits strong dehydration and oxidation properties. The ester groups in the PET molecules are rapidly hydrolyzed under the action of concentrated sulfuric acid, the molecular chains are gradually broken, and the originally high molecular weight PET long chains are decomposed into small molecular fragments.

This causes the mechanical properties of the PET material to drop sharply, and the originally tough PET plastic bottles will become fragile and brittle, with reduced transparency, and cracks and holes on the surface. If PET fibers are placed in a high temperature and strong acid environment, the strength and elasticity of the fibers will be rapidly lost, and they will no longer be able to meet the use needs of the textile industry. ​

3. Degradation of PET by high temperature and strong alkali ​

In a strong alkaline environment, high temperature will also accelerate the degradation of PET. Taking sodium hydroxide (NaOH) as an example, high temperature greatly increases the activity of hydroxide ions (OH⁻), which can quickly react with the ester groups in the PET molecular chain to induce alkaline hydrolysis. 

As the reaction proceeds, the PET molecular chain continues to break, producing small molecules such as terephthalate and ethylene glycol. This reaction is more intense at high temperatures, and will cause serious damage to PET materials in a short period of time.

For example, PET plastic products immersed in high-temperature sodium hydroxide solution will expand in volume and become rough on the surface after a few hours. Long-term immersion may even cause the material to completely dissolve. ​

4. safety and environmental issues caused by the reaction​

In addition to the direct damage to the material properties, the reaction of strong acids and alkalis with PET at high temperatures may also cause safety and environmental problems.

The small molecules produced by the decomposition of PET under the action of strong acids and alkalis are partially toxic and corrosive. If these substances enter the environment without proper treatment, they will pollute the soil and water.

In industrial production, if PET equipment is used to store or transport high-temperature strong acid and alkali solutions, once the equipment is corroded and perforated, it will not only cause strong acid and alkali leakage, causing safety accidents such as burns, but also cause production interruption and huge economic losses. ​
Five, protection strategies for high-temperature strong acids and alkalis​

In order to avoid damage to PET by strong acids and alkalis at high temperatures, a series of effective measures need to be taken in practical applications. During the product design phase, materials should be carefully selected according to the usage scenario to avoid direct contact between PET and high temperature, strong acids and alkalis.

If contact cannot be avoided, PET can be surface modified, such as applying a layer of acid-resistant and alkali-resistant protective coating to enhance its corrosion resistance. In industrial production, PET equipment that may be exposed to high temperature, strong acids and alkalis must be regularly inspected and maintained, and leakage monitoring devices must be installed.

Once abnormalities are found, they must be handled promptly. At the same time, companies should strictly abide by environmental protection laws and regulations, and harmlessly treat wastewater and waste residues containing PET decomposition products generated during the production process to reduce the impact on the environment. ​

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