How Air Pollution Impacts Rubber Deterioration

Rubber pollution is a pressing issue that affects various environments, from the air we breathe to the soil and waterways. While tyre wear is a significant contributor to rubber pollution, it is not the only culprit. Artificial turf, rubber O-rings, and seals are also sources of rubber pollution. The production and use of tyres generate multiple heavy metals, plastics, and other toxic compounds, which can have detrimental effects on both the environment and human health. Tyre wear particles (TWPs) are released into the environment during vehicle transit, and the addition of antiozonants in rubber tyres has been linked to the death of salmon. With approximately 3 billion new tyres produced annually and 800 million becoming waste, the need for comprehensive clean-up solutions and effective management strategies is more urgent than ever. Aside from pollution, rubber degradation can occur due to environmental factors such as heat, light, and ozone, leading to molecular changes that impact the mechanical properties and service life of rubber products.

Ozone, light, and heat cause rubber degradation

Rubber degradation occurs due to common environmental factors. Oxidative and thermal ageing of rubber are accelerated by stress and reactive gases, like ozone, resulting in cracking, charring, and colour fading.

Ozone is present everywhere, even in "normal" air, which can contain up to 0.01 ppm of ozone. The presence of electrical equipment or lighting can increase ozone levels. Different rubber polymers have varying resistance to ozone. Less saturated rubber polymers are more susceptible to ozone due to their higher number of double and triple chemical bonds, which are vulnerable to the free radicals created by ozone. This results in the breakdown of these bonds, causing cracking, a process known as dry rotting. Test methods like ASTM D1171 help classify rubber behaviour when exposed to ozone and can be used to predict the growth rate of cracks.

Light exposure can also cause oxidative rubber degradation, leading to hardening or softening, depending on the elastomer structure. Light exposure produces free radicals, which combine to form new crosslinks, reducing the flexibility of the rubber and causing hardening, known as chain hardening.

Heat exposure can cause thermal expansion in most materials, but rubber exhibits unusual behaviour due to its polymer structure. When heated, the long polymer chains vibrate more intensely, causing them to shorten and resulting in the contraction of the material. This phenomenon is the opposite of what is typically expected, as most materials expand when heated.

To mitigate rubber degradation, rubber products should be stored in cold, dark, dry, and oxygen-free conditions. Certain rubber restoring techniques can also be employed to repair damaged or weathered rubber seals and restore their functionality.

Rubber tyre wear is a major source of air pollution

The composition of tyres is a crucial factor in pollution. Certain chemicals used to prevent the degradation of rubber in tyres are known to be ecotoxic, yet they are still used. For example, 6PPD, a chemical added to tyres to prevent cracking and degradation, can be transformed into 6PPD-quinone when exposed to ground-level ozone. 6PPD-quinone has been found to be toxic to salmon when it accumulates in waterways from tyre wear pollution.

Tyre wear particles emitted during vehicle transit pose a significant environmental and human health risk. These particles are so small that they can be inhaled and travel directly to the brain, or pass through lung tissue into the bloodstream, potentially causing a range of health problems. In addition, tyre wear contributes to the microplastic pollution problem. Microplastics from tyre wear have been found to affect not only the environment but also wildlife, including salmon.

To address the issue of tyre wear pollution, policymakers must implement stricter chemical requirements for tyre manufacturers. Banning tyres with high tread abrasion and adopting international standards for measuring tyre abrasion can help reduce microplastic emissions from tyres. Comprehensive cleanup solutions and management approaches are also necessary to reduce the risk of tyre wear pollutants to human health and the environment.

Tyre emissions include toxic heavy metals

Tyre emissions are a significant source of air and water pollution, and they may be affecting humans as well as wildlife. Tyre wear particles (TWPs) are released into the environment at a rate of 6 million tons per year, with the United States and the European Union emitting roughly 1,120,000 and 1,327,000 tons annually. Cars in the US emit, on average, 5 pounds of tyre particles per year, while cars in Europe shed 2.5 pounds per year due to fewer miles driven. Tyre emissions from electric vehicles are 20% higher than those from fossil-fuel vehicles because electric vehicles weigh more and have greater torque, which wears out tyres faster.

Tyres are made from about 20% natural rubber and 24% synthetic rubber, with the remaining composition consisting of hundreds of other ingredients, including steel, fillers, oils, vulcanisation chemicals, and heavy metals such as copper, cadmium, lead, and zinc. These heavy metals are added to enhance performance, improve durability, and reduce the possibility of fires. As tyres break down, they release a range of particles, from visible pieces of tyre rubber to nanoparticles. Large particles are carried from the road by rain into rivers, where they may leach toxic chemicals into the environment, while smaller particles become airborne and are small enough to be breathed deep into the lungs.

The toxic chemicals released from tyre emissions include polyaromatic hydrocarbons, benzothiazoles, isoprene, and heavy metals. Tyre wear particles are a significant source of microplastics in rivers and oceans, with tyre wear in cities posing up to a four-fold greater risk to the environment than other microplastics. Seventy-eight percent of ocean microplastics are synthetic tyre rubber, according to a report by the Pew Charitable Trust. These fragments are ingested by marine animals, causing neurotoxicity, growth retardation, and behavioural abnormalities.

In addition to the heavy metals mentioned above, tyre dust contains more than 400 chemicals and compounds, many of which are carcinogenic. One of these chemicals is 6PPD, which is added to tyres to prevent cracking and degradation. When exposed to ground-level ozone, 6PPD transforms into multiple other chemicals, including 6PPD-quinone (6PPD-q), which has been found to kill salmon when it accumulates in waterways from tyre wear pollution. The acute toxicity of 6PPD-q has been well established, and researchers are now working to understand the full extent of the toxic cocktail of chemicals, tiny particles, and heavy metals hiding in tyres.

Rubber dust particles are airborne pollutants

Airborne dust particles, also known as particulate matter (PM), are tiny solid particles or liquid droplets that float in the air. They are often invisible to the naked eye, but they can cause haze when they accumulate and block the spread of sunlight. PM can be directly emitted from sources like combustion processes or formed in the atmosphere through chemical reactions. Rubber dust particles fall under this category and can have adverse health effects when inhaled.

The size of rubber dust particles can vary, with fine rubber dust particles being classified as microplastics due to their similarity to plastic polymers. These fine particles can be inhaled and cause damage to the respiratory system. They can pass through the nasal passage and reach the deepest parts of the lungs, leading to tissue damage, inflammation, and increased risk of respiratory diseases. People most vulnerable to the effects of breathing rubber dust particles include children, the elderly, and individuals with pre-existing respiratory or heart conditions.

Additionally, rubber dust particles can have ecological impacts. For example, 6PPD-quinone, an antiozonant used in rubber tires, has been found to contaminate waterways and harm aquatic life, such as salmon. The accumulation of rubber dust in the environment can also contribute to soil and water pollution, affecting ecosystems and human health.

To address the issue of rubber dust particles as airborne pollutants, regulatory bodies like the United States Environmental Protection Agency (EPA) have implemented standards for hazardous air pollutants. These standards aim to reduce emissions and protect human health and the environment from the adverse effects of rubber dust and other hazardous pollutants.

Natural rubber contributes to deforestation

Natural rubber is made by "tapping" a sticky sap called latex, which is collected by making incisions in the bark of certain types of trees. Predominantly, this is the Hevea brasiliensis tree, a deciduous species native to the Amazon basin that is now found across the tropics. Today, Southeast Asia accounts for 90% of global rubber production, with the remaining 10% coming from South and Central America and, more recently, West and Central Africa.

While natural rubber is a renewable resource with the potential to contribute to climate change mitigation, its production can lead to severe negative consequences if not carefully managed. The expansion of rubber plantations has been linked to substantial deforestation, particularly in tropical regions. This has resulted in biodiversity loss and net carbon emissions.

High-resolution satellite imagery has revealed that rubber plantations have expanded significantly since 2010, often at the expense of natural forests. The distinct seasonal signature of rubber plantations, based on when they shed and regain their leaves, has helped distinguish them from adjacent forests in satellite images. This technology, combined with cloud computing and case study analyses, has provided more accurate data on the impact of the global rubber trade on forests.

The inclusion of rubber in zero-deforestation legislation, such as the EU Deforestation Regulation, is necessary to address the environmental impact of rubber production. While soy and palm oil have contributed to more deforestation than rubber, the latter has received less attention in policies due to its relatively minor impact. However, with the increasing demand for natural rubber, ensuring sustainable practices becomes crucial to prevent further environmental degradation.

To mitigate the impact of rubber production on deforestation, it is essential to educate farmers on sustainable practices, such as minimizing tree tapping and forming cooperatives to reduce pesticide use. Additionally, governments should optimize land use to balance agricultural production and environmental conservation. While natural rubber provides significant income and climate benefits for smallholder farmers, sustainable practices and regulations are vital to ensure its production does not contribute to continued deforestation.

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