Plastic is one of the most versatile and commonly used materials in modern society. However, plastic does not easily break down and can persist in the environment for hundreds of years. Interestingly, plastic can and does get smaller over time through various chemical and physical processes.
Weathering of Plastic
One way plastic gets smaller is through weathering. Weathering is the process by which plastic is broken down into smaller pieces through exposure to sunlight, rain, wind, and other environmental conditions over time. Ultraviolet light from the sun is particularly effective at breaking chemical bonds in plastic, making it more brittle and causing it to flake apart into small fragments called microplastics.
Photodegradation
Photodegradation is the specific process by which ultraviolet light breaks down plastic polymers. When UV light hits plastic, it excites electrons in the polymer chains of the plastic. This makes the plastic unstable and causes it to break into smaller molecular chains. Over time, the plastic breaks down into tiny microscopic pieces of plastic debris.
Thermooxidative degradation
Heat and oxygen can also degrade plastics in a process known as thermooxidative degradation. Plastics are subject to heating and cooling cycles in the environment which cause the material to expand and contract. Over time, this creates fractures and cracks in the plastic. Oxygen reacts with the broken polymer chains, leading to further degradation.
Hydrolysis
Water can also degrade plastic through hydrolysis reactions. When water molecules come into contact with the polymer chains in plastic, it can result in hydrolysis which breaks the bonds between the polymer chains. This makes the plastic more brittle and causes it to break apart faster.
Biodegradation
Some special types of plastic can be biodegraded by microorganisms. For example, PLA (polylactic acid) plastic can be broken down by bacteria and fungi. As microorganisms digest the plastic, it breaks down into smaller compounds, eventually becoming basic organic and inorganic molecules. However, most conventional plastics are not biodegradable.
Mechanical Breakdown of Plastic
In addition to chemical weathering processes, plastic can also be physically broken down into smaller pieces through mechanical forces.
Wave Action
Wave action along shorelines grinds plastic debris against sand and rocks, breaking it into smaller and smaller fragments. Over time, waves can reduce plastic items like bottles or fishing nets into tiny microplastics.
Abrasion
Plastic is also abraded as it is transported by wind and water currents. Bouncing along the ground or scraping against other debris causes surface scratches, fractures, and breakdown of plastic particles.
Thermal Stress
Heating and cooling cycles in the environment cause thermal stress and expansion/contraction in plastics. This physical stress causes cracks and fractures that break plastic into smaller pieces.
Reduction to Microplastics and Nanoplastics
The weathering and mechanical breakdown processes gradually reduce plastic waste into tiny plastic particles. Microplastics are plastic fragments smaller than 5 mm in size, while nanoplastics are particles smaller than 0.1 μm across. These microscopic plastic particles are abundant ocean pollutants and can accumulate up the food chain.
Common Microplastics
- Microbeads – used in cosmetics and personal care products
- Synthetic fibers – from fabrics and clothing
- Tire dust – abrasion of car tires on roads
- Plastic pellets – used in plastic manufacturing
- Fragmented plastic – from broken down plastic debris
Factors Affecting Plastic Breakdown Rate
Certain conditions cause plastic to break down into smaller pieces more rapidly:
- Exposure to sunlight
- Heat, freezing temperatures, and temperature fluctuations
- Wave action and mechanical forces
- Acidic or basic environments
- Presence of oxygen
- Microorganisms and biofilm activity (for biodegradable plastic)
Denser plastics like HDPE break down slower compared to plastic bags. Plastics in deep ocean environments break down slower due to lack of UV exposure, oxygen, and mechanical forces.
Persistence of Plastic Particles
While plastic debris gradually breaks down into smaller and smaller pieces, it may never fully biodegrade. Plastic contains stable carbon-carbon molecular bonds that are resistant to complete breakdown. Many microplastic particles resist further breakdown and persist for incredibly long timespans.
Research models suggest plastic takes hundreds of years to degrade completely, though plastic persistence depends on environmental conditions. Plastic debris will likely remain present in the ocean for centuries.
Plastic Breakdown Effects on the Environment
The breakdown of plastic waste has a number of concerning effects on ecosystems:
- Microplastics and nanoplastic particles are ingested by marine organisms, accumulating up the food chain
- Chemical additives like BPA that leach from plastics can harm organisms
- Plastics concentrate and transport other pollutants in the environment
- Plastic breakdown products remain present for centuries after use
Since plastic breaks down so slowly, it is critical to reduce plastic waste pollution through recycling programs, bans, and biodegradable alternatives. Though plastic gets smaller over time, it still remains a pervasive and problematic environmental pollutant.
Conclusion
Plastic breakdown occurs through weathering processes like photodegradation, hydrolysis, and biodegradation which break plastic polymer chains. Mechanical forces like wave action and thermal stress also physically degrade plastic over time. These processes gradually reduce plastic items into microscopic particles like microplastics and nanoplastics which persist for incredibly long periods of time. Understanding how plastics break down can help guide efforts to mitigate plastic pollution through proper disposal, recycling, and material innovations.