What is the hydrolysis of plastics?

Plastics degrade in water by hydrolysis. Water molecules are able to react with specific chemical bonds in the polymer chains to form new molecules.

These new molecules are usually much shorter in length. This process occurs in several stages, as shown in the figure below.

Hydration stage (stage 1): The aqueous medium penetrates the polymer matrix and disrupts secondary forces. This causes the polymer to relax and lower its glass transition temperature.

In this initial stage, water molecules act like tiny wedges that gradually penetrate into the complex network structure of the polymer matrix. Secondary forces such as hydrogen bonds and van der Waals forces that hold the polymer chains in a relatively ordered state are weakened by the intrusion of water. 

Just like a neatly stacked stack of cards is gently disturbed, the polymer chains begin to move more freely, which leads to the relaxation of the material structure.

As a result, the glass transition temperature (the temperature at which the polymer changes from a hard glassy state to a more flexible rubbery state) is reduced. This makes the polymer more susceptible to further chemical and physical changes because the polymer chains are now more mobile and more accessible to other reactants. 

For example, in humid environments, polyamide-based plastics hydrate relatively quickly, and a decrease in their glass transition temperature can be observed in a short period of time, which in turn changes the mechanical properties of the material at room temperature.

Initial degradation stage (stage 2): In areas where the polymer is hydrated, the covalent bonds in the polymer backbone begin to break.

This results in a decrease in the molecular weight of the polymer. As the hydrolysis proceeds, the hydrolysis reaction within the polymer matrix is ​​autocatalyzed by an increasing number of carboxyl end groups, causing the molecular weight of the polymer to continue to decrease. 

As the molecular weight decreases, the polymer also loses its mechanical strength, but the polymer still maintains its integrity. When the covalent bonds in the polymer backbone (such as ester bonds, amide bonds, or carbonate bonds) come into contact with water molecules, a chemical reaction occurs.

The water molecules break apart and a hydrogen atom is attached to one part of the polymer chain and a hydroxyl group to another part, effectively breaking the covalent bond. 

For example, in polyester-based plastics such as polyethylene terephthalate (PET), ester bonds are particularly susceptible to hydrolysis. As these bonds break, long polymer chains break into shorter chains, and the molecular weight decreases.

The newly formed carboxyl end groups catalyze further hydrolysis reactions. This self-accelerating process means that once hydrolysis begins, its rate tends to increase over time. As molecular weight decreases, the polymer's ability to resist external forces, such as tension or pressure, decreases. However, the material still retains its overall shape, albeit becoming more flexible and less durable.

Further degradation (Stage 3): The molecular weight of the polymer continues to decrease until a threshold is reached where the polymer can no longer maintain its integrity and begins to experience noticeable mass loss. As the hydrolysis reaction continues and the molecular weight decreases further, the polymer reaches a critical point.

The remaining molecular chains become too short and too weak to maintain the structural integrity of the material. This is like a rope that has worn and broken into so many small pieces that it can no longer support any weight. At this stage, the polymer begins to break down into smaller pieces.

For example, a plastic film that is undergoing hydrolysis may begin to develop small holes and the edges may begin to crumble. These pieces are then further broken down by other environmental factors, such as water flow or microorganisms, resulting in noticeable mass loss.

The polymer, which once had a specific shape and size, begins to disintegrate and become more dispersed in the environment.

Dissolution or erosion phase (Phase 4): The mass of the polymer is reduced and its fragments are further broken down into molecules that are soluble in the medium. In this final phase, the small fragments of the polymer break down into even smaller molecules and dissolve in the water.

These soluble molecules are then carried away by the water flow, causing the plastic to disappear from its original location. This process is similar to the dissolution of sugar in water.

Taking plastics as an example, substances such as lactic acid oligomers that may be formed during the hydrolysis of polylactic acid (PLA) are able to dissolve in water and be transported to other parts of the environment.

This not only leads to the complete disappearance of the original plastic material, but also has an impact on the surrounding ecosystem. 

These dissolved molecules may be absorbed by aquatic organisms, potentially causing harm through bioaccumulation, or interfering with normal biological processes.

The rate and extent of plastic hydrolysis are affected by many factors. Temperature plays a key role, and generally the higher the temperature, the more energy is provided for the chemical reaction, and the faster the hydrolysis reaction.

The pH of the water also affects the hydrolysis process, and some plastics degrade faster in acidic or alkaline environments.

In addition, the presence of natural or synthetic catalysts can accelerate the hydrolysis process. Understanding the hydrolysis of plastics is crucial for developing management strategies for plastic waste in aquatic environments, such as designing more biodegradable plastics or improving waste treatment processes to prevent the release of harmful degradation products.

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