Polymer degradation is the process where the large molecules in plastics break down into smaller fragments. This breakdown causes a loss of the material’s original properties, such as its strength and flexibility. The process is like breaking the links in a long chain, creating shorter, weaker pieces. As this happens, a once tough material becomes brittle and fragile, losing the characteristics that made it useful.
Causes and Processes of Degradation
The triggers for polymer degradation are environmental forces that assault the chemical structure of these materials.
High temperatures can cause thermal degradation, a common issue for plastics in electronics or cars. The heat makes polymer chains vibrate intensely enough to break their chemical bonds in a process called chain scission. This creates highly reactive fragments called free radicals that can perpetuate further breakdown.
Photodegradation is driven by light, particularly the UV radiation in sunlight, which is why plastic items left outdoors become brittle. The absorbed energy can sever chemical bonds and form free radicals. These radicals start a chain reaction, attacking polymer chains and spreading damage. This process is often accelerated by oxygen in an effect called photo-oxidation.
Oxidation is a chemical reaction where oxygen attacks the polymer’s molecular backbone. While it does not require light, the process is often accelerated by heat. Oxygen molecules react directly with the polymer chains, introducing new chemical groups that weaken the structure and make it more prone to breaking.
Hydrolysis is a degradation process involving water. Water molecules can chemically react with and split certain polymer chains, especially in materials like polyesters and nylons. This is a chemical severing of the polymer links by water, not just the material getting wet. It is a concern for plastics used in humid environments or those with constant water contact.
Biodegradation is the breakdown of polymers by living organisms. Microorganisms like bacteria and fungi secrete specialized enzymes that can latch onto the polymer chains and break them down into smaller molecules. The microbes then absorb these smaller fragments as a source of carbon and energy, effectively recycling its components back into the ecosystem.
Environmental and Material Consequences
The most direct consequence of degradation is a loss of mechanical integrity. As the long polymer chains are broken into smaller segments, the material becomes brittle and loses its toughness and tensile strength. This can lead to failures in products, such as a plastic component in a car suddenly fracturing or a pipe becoming too fragile to withstand pressure.
Another frequent consequence is discoloration. The chemical changes that occur during degradation, especially in photo-oxidation, alter how the material interacts with light. This commonly results in a yellowing or browning effect, a clear visual indicator that the polymer’s structure has been compromised. The surface may also develop cracks or a chalky appearance.
A significant environmental consequence of polymer degradation is the creation and spread of microplastics. As larger plastic items like bottles and bags are exposed to sun, water, and mechanical stress, they fragment into progressively smaller pieces. This process continues until the particles are less than five millimeters in size, creating a pervasive form of pollution.
These tiny plastic particles are now found in virtually every ecosystem on Earth. Their small size allows them to be easily ingested by wildlife, potentially moving up the food chain. As plastics break down, they can also release chemical additives, such as plasticizers or flame retardants. These substances can then leach into the surrounding soil and water, contributing to chemical pollution.
Managing Polymer Lifecycles
Controlling polymer degradation involves both preventing it for durability and designing it for disposal.
Preventing Degradation
For products that need to last, such as automotive parts or window frames, manufacturers incorporate specific additives into the plastic. UV stabilizers are compounds that absorb ultraviolet radiation and dissipate it as harmless heat, protecting the polymer from photodegradation. Antioxidants are molecules that interrupt the oxidation process by neutralizing the free radicals that would otherwise attack the polymer chains.
Heat stabilizers are also used to protect polymers during high-temperature processing and throughout the product’s service life. By carefully combining these protective compounds, engineers can design plastic materials that resist environmental pressures and maintain their structural integrity for decades.
Designing for Degradation
Conversely, a growing field focuses on designing polymers meant to degrade in a controlled and timely manner. This is particularly useful for single-use items like packaging, agricultural films, and disposable cutlery. These biodegradable plastics are formulated to be susceptible to hydrolysis or microbial action, allowing them to break down completely in specific environments like an industrial composting facility.
This concept of designed degradation extends into advanced medical applications. Dissolvable sutures, for instance, are made from polymers that are engineered to break down via hydrolysis inside the human body as the tissue heals. Similarly, some drug delivery systems use biodegradable polymers to create capsules that release a medication over time as the polymer matrix slowly degrades, making degradation a precisely engineered function.