Researchers are now referring to the extensive use of plastics as a “global plastic cycle” equivalent to the well-documented “carbon cycle” – both of which are directly related to the built environment we humans have developed. Because people literally breathe, eat and drink microplastics, their rising levels are not surprising, and are nothing short of alarming given their associated health and ecological risks.   

Social concerns about how to better manage plastic consumption have become almost as universal as concerns over carbon reduction management. For example, plastic straws have become a target on the radar of many consumer organizations and consumers themselves. But are straws really the biggest problem? 

The Role of Vehicle-Related Microplastics

What many may not realize is that as much as 85 percent¹ of the microplastics that end up in our environment come from roads. The culprit is cars; both tire and brake-pad wear are the significant contributors.   

Scientists have even come up with a name for roadway microplastic accumulations: “road-associated microplastic particles” aka RAMPs.  Tire-wear particles are considered the leading source.

Test Results Show Near 100% Reductions

Several years ago, Pavement Technology, Inc. along with the Texas A&M Transportation Institute (TTI), embarked on the development of a novel technology to sustainably introduce photocatalytic-grade titanium dioxide (TiO₂) to pavements by combining titanium with the company’s long-proven, pavement-penetrating preservation technology. 

Initial results were beyond earliest forecasts. In the field, roads retrofit with photocatalytic penetrants are showing as much as 50 percent vehicular NOx removal, while more than tripling solar reflectance index (SRI) values, thereby mitigating Urban Heat Island effect (UHI).  

The Texas A&M Transportation Institute (TTI) has also conducted extensive tests on the efficiency of TiO₂ of microplastics decomposition. In other independent studies, photocatalytic-grade titanium has proven to remove 98 percent of plastics², including but not limited to the various forms of microplastics found in RAMPs.  TTI’s testing of PTI’s PlusTi family of photocatalytic technologies has produced similar results as indicated in the table below. 

TTI’s latest research shows that the photo-reactive nature of titanium provides another strong environmental benefit by naturally reducing microplastic accumulations from tire and brake-pad wear. In the United States alone, the added microplastic load from on-road non-exhaust emissions is equivalent to 450 billion plastic water bottles in relative plastic pollution removal potential. Removing such pollutants is critical to preserving the health of human and marine life by sustaining a sound and resilient water ecology.  

The United Nations Environmental Program is currently developing an international legally binding instrument to reduce plastic pollution as a growing environmental hazard. These on-road non-exhaust emissions are also part of the Federal Highway Administration’s (FHWA) “Climate Challenge” initiative, which is expected to play a large role in future highway construction. 

Conclusion

Plastics have half-lives in the hundreds of years. If left “’spiraling around the globe” atmospheric RAMPs will remain free to invade our air, contributing to the hazards related to a global plastic cycle. Scientists have sounded the alarm. The transportation industry is listening. 

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Let us explore how PCT works in the roadway microenvironment and imagine its monumental implications for the transportation industry and societal climate prioritization.

How the Photocatalytic Process Works

Once energized, TiO₂ releases electrons into the surrounding area (up to six meters in all directions), which then combine with oxygen in the air to create the superoxide anion (O₂-). Simultaneously, the catalyst TiO₂ replaces its lost electrons by attracting electrons from ambient humidity, and then returns to its original state unaffected by the photocatalytic process. The moist air that has lost electrons transforms into hydroxyl radicals (-OH). Superoxide and oxygen radicals, collectively “oxygen radicals,” yield strong oxidative decomposition.

In the case of vehicular emissions, millions of oxygen radicals, created by the TiO₂ solar reactivity, attack “bent” or vulnerable compounds such as nitrogen oxides (NOx), carbon dioxide (CO₂), and volatile compounds (VOCs) created by fuel combustion. A bent compound has an imbalanced molecular structure in which electrons are underweighted on one side of the structure. The superoxide and oxygen radicals thus penetrate the vulnerable side of the alien molecule and instantly alter its molecular structure in a manner beneficial to the environment. Photocatalytic oxidation is nature’s self-cleaning mechanism and a photo-responsive material like TiO₂ accelerates the process.

For NOx, oxygen radicals transform toxic airborne nitrite gas (NO₂), which is quite harmful to the human respiratory function, into grounded harmless nitrate (HNO₃), interrupting and precipitating the nitrogen cycle. Disposal of the sequestered HNO₃ by natural consumption or dilution adds nothing to the overall nitrogen load. Road level PCT hence elegantly eliminates the most dangerous part of the cycle by merely accelerating essential detoxification at the point source. It’s like instantly disarming vehicular exhaust.

Other vulnerable compounds such as water (H₂O) do not fully transform into a new molecular phase like NOx, and instead transform into a differently shaped molecule in a manner that is similarly environmentally beneficial. For example, agglomerated water molecules in the form of rain, ice or snow transform into thin films, reducing water build-up by hastening precipitation disposal. This improves rainstorm roadway safety and deters ice or snow formation in colder climates.

A surface course such as a roadway embedded with photo-reactive titanium creates a fully hydroxylated surface that perpetually creates oxygen radicals when irradiated by sunlight. The use of photocatalytic construction materials in our built environment can amplify and accelerate these natural air-purifying processes in places where the altered landscape reflects human consumption.

Improving Near-Roadway Microenvironments

As the most highly efficient photocatalyst, TiO₂ is a multifaceted photo-responsive mineral¹ rapidly gaining increased scientific and commercial interest for near-roadway microenvironments (ME) by advancing a host of preservation, resilience and environmental benefits, including:

• Depolluting near-pavement air-cleaning applications, where TiO₂-reacted surfaces are able to oxidize a variety of pollutants and contaminants, such as those emitted by vehicles (especially NOx and VOCs), thereby reducing ozone pollution and mitigating acid rain formation.
• Producing cool pavement applications in which TiO₂-treated surfaces provide a solar-reflective top boundary. This lessens pavement-related radiative forcing (RF) by reducing the convective re-release of solar radiation that leads to Urban Heat Island effect (UHI), slowing oxidation² to enhance the life-cycle assessment of pavement.
• Promoting more hydrophilic or rapid water-desorbing pavements that are self-cleaning, resist staining and remove contaminants such as mold. Hydrophilic pavements protect against water intrusion for extended pavement life and promote safer roads during poor weather conditions by displacing rain and inhibiting ice formation³.
• Decomposing vehicular microplastic debris by as much as 98 percent with photo-reactive TiO₂ ⁴. Dangerous airborne and oceanic microplastic pollution accumulations are estimated to be as much as 75 percent⁵ sourced from roadway tire and brake-pad wear.

Tested and Proven Performance

Pavement Technology, Inc., in collaboration with the Texas A&M Transportation Institute, embarked on the development of a novel technology to sustainably introduce photocatalytic-grade TiO₂ to pavements by combining TiO₂ with our long-proven pavement penetrating and preservation technologies. The results have been beyond our earliest forecasts.

In the field, roads retrofit with our family of PlusTi^TM photocatalytic penetrants are showing vehicular NOx removal as high as 60 percent and have tripled the solar reflectance values of treated pavements, thereby mitigating the effects of UHI.

A TiO₂-Enhanced Roadway’s “Zone of Influence”

Engineers use the concept of “zone of influence” when describing the impact(s) on or from a structure from all dimensions. In this sense, a photocatalytic pavement is a three-dimensional structure reacted through a catalyst material embedded near the surface and by the power of the sun from above.

A TiO₂-bearing roadway creates a natural electrochemical energy field that spans about eighteen feet in all directions from the roadway surface. This zone of influence acts like a tunnel of perpetual solar-charged particles that surround vehicles traveling across the roadway as if they were passing through a self-cleaning, photo-catalyzed subway.

As described, certain vehicular-introduced compounds including hazardous air pollutants (HAPs), such as NOx and other Ozone (O₃) precursors, become captured and scrubbed (i.e., rendered harmless) before they can escape beyond the zone of influence.

Carbon Reduction Calculations

In addition to our Maltene Replacement Technology (MRT) life-cycle cost calculator, which confirms a 500-to-1 carbon savings in comparison with rebuilds⁶, PTI has developed a carbon-capture model for our photocatalytic pavement solutions with the help of Texas A&M, the National Center for Pavement Preservation at Michigan State, Alcoa Architectural Coatings, and the U.S. Forestry Service, et al.

One mile of TiO₂-enhanced pavement is “valued” at the same carbon reduction capacity as an approximately 15-acre forest. That 15-acre-forest equivalent pavement, using a conservative, discounted weighted average ten-year service

life, can offset the pollution of as many as 375 automobiles or more than 4,000 tons of carbon equivalents annually.

Applied to the Paris Agreement 2030 carbon pricing model [$50-$100 per ton CO₂e], one mile of PlusTi-enhanced roadway has a net present value of $75,000, which is three-times the current estimated capital cost of the upgrade. In direct financial cost, a unit of carbon removed via a PCOPCT-enhanced pavement costs only 19c ($1 per car per year), while other carbon-capture technologies are estimated to cost as much as $200 or more per unit of carbon removed. ⁷

Photocatalytic pavements are highly productive at removing carbon because the primary mobile-sourced pollutant, i.e., NOx – is 298 times the environmental value (toxicity) of a unit of CO₂ per the EPA and the Clean Air Act (CAA).

Conclusion

Roadway microenvironments are a critical and advantaged target for agencies promoting more aggressive carbon neutral priorities. Pavement preservation programs leveraging PCT are now leading the way toward a cleaner, healthier low-carbon future.

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These problems will require an array of diverse viewpoints, creativity and innovation to solve.  

Changes in how we design, build, preserve and even view infrastructure will play a critical role in implementing the solutions.

Pavement Technology, Inc. was started in the wake of the first Earth Day by my father and grandfather to promote sustainable pavements.  Nearly fifty years later, our pavement preservation technologies are widely used because they reduce road rebuilds by half a cycle to a full cycle and often longer.  

Today, our revolutionary Photo Catalytic Technology solutions for pavement preservation not only help roads last a lot longer; they also:

• Eliminate vehicular pollution
• Reflect heat to mitigate the Urban Heat Island effect
• Decompose roadway microplastics
• Provide a super-hydrophilic surface that dissipates surface moisture more rapidly

Many view the transportation challenges related to climate change as an intractable problem. But what if we could flip the script and turn our vast transportation infrastructure into a weapon against climate change?  Pavement Technology not only has this on our minds, but it is also now in our practice.  

Being in the pavement business, one of our favorite metaphors is where is the road taking you?   If you are asking us today, it is taking us to a cleaner, healthier, happier and more productive place, one smog-eating road at a time.  

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PTI is proud to announce it has become a sustaining partner of the Global Cool Cities Alliance’s (GCCA) Cool Roadways Partnership. Learn how GCCA and the Cool Roadways Partnership is helping accelerate a world-wide transition to cooler and healthier cities.

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Happy 50th Anniversary Earth Day from Pavement Technology! Pavement Technology, Inc. (PTI) is disappointed to be unable to celebrate the 50th Earth Day with our friends at EarthX2020 due to COVID-19. But we are all thankful our nation has embraced the challenge, and the curve has started to bend – that’s what matters most.

Just the same, the world’s conservationists and environmentalists, PTI included, will be sharing many virtual Earth Day celebrations on Wednesday, April 22nd.

Meanwhile, the pandemic crisis is showing us all just how critical urban smog has become in our built world, as demonstrated by its absence during the current shut downs. From the mountain peaks of the Himalaya, which have not been visible in decades…to the fish feeding in the now-clean canals of Venice…to the dramatic NASA Earth Observatory photos of our greatest metropolises – we can’t help but wonder at the irony of all of this occurring while we are celebrating Earth Day’s 50th anniversary.

Today, our great challenge is to improve our impact on the planet, without reversing the advance of human progress. That challenge is one we can all too comfortably ignore during normal times. But the stark before-and-after evidence of the near-universal lockdowns on air quality is setting society’s stretch goal of what’s achievable. It should motivate us towards minimizing its impact through corrective action.

PTI is proud of the contributions our company has made towards that end. We started out just about the same time as Earth Day, at the dawn of the environmental age, with sustainable pavement science that conserves natural resources through roadway preservation. Today, our latest technology is targeting the effects of vehicular pollution and Urban Heat Island (UHI) at the critical roadway point source. A single mile of our “smog eating roads” can consume as much carbon as a 20-acre forest of trees.

It’s no coincidence that Earth Day and PTI are sharing a golden anniversary – Happy 50th!

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Scientists have noted that worldwide quarantines have dramatically reduced ground-level air pollution. In Paris, nitrogen dioxide readings have dropped a stunning 60 percent in the past few weeks. But what we’ve also learned in this pandemic is that austerity and abstinence, the long-offered solution to urban smog, will have a devastating impact on the human condition.

Unemployment claims in the United States are setting records, with nearly 10 million job losses in just the first weeks of reduced activity. It’s now obvious: the cure is greatly out-damaging the disease – at least in the short-term.

Therapeutic and preventive medicine is how we’ve managed life-threatening diseases historically. Today’s directives for the cessation of most business activity ad infinitum, is resulting in extreme human hardship. Yet we persist in presuming that draconian business restrictions are the solution for managing pollution.

Taxing human activity – the commonly proposed panacea for managing pollution – has failed miserably when tried. Carbon tax schemes, in this sense, were a dress rehearsal for what would happen to our economy should a pandemic occur. Now that we are all witnessing first hand the human sacrifice required by a near-total cessation of commerce, do we really believe inflicting that penalty on our business community is the best approach to reducing pollution?

Some would argue renewable energies are the answer to pollution reduction. But if that’s the case, why do renewable solutions remain almost 100 percent reliant upon tax credits, government subsidies, and utility conversion mandates?

In Australia, a 2011 carbon tax coupled with mandatory energy grid conversion resulted in a tripling of utility bills for Australians. If that wasn’t hardship enough, the country’s “sacrifice” only managed to achieve 6 percent renewable energy. In 2014, the Australians repealed the tax via an electoral overhaul of their government – in a landslide.

Which brings us back to the sacrifices we are all undertaking to suppress today’s pandemic. Outright cures are very rare in human history. For the most part, we manage disease with prevention and symptom management – and then we adjust and carry-on. This will likely prove true for COVID-19, and it is a lesson that is equally applicable to the pollution challenges we are facing.

Single vehicle emissions have declined over 70 percent since the 1970s. We didn’t “cure” for the unintended consequences of fuel combustion; instead, we managed the impact. The breakthrough was the invention of the catalytic converter, an early form of carbon capture technology. To date, it has been the only scalable success story in vehicle emissions-related pollution mitigation. All other strategies have failed, including public transit, high occupancy vehicle lanes and electric vehicles. Ironically, the latter produces as much as 50 percent more pollution than traditional vehicles due to their energy-squandering production process and high pollution energy sourcing.

Still, the rampant urbanization of our life style choices has superseded the catalytic converter. While cars run significantly cleaner than in decades past, we are adding more vehicles so quickly to the grid that we’re unable to bend the curve for air quality. New, complementary technologies are needed.

On-road emissions represent as much as 80 percent of urban smog, principally nitrogen oxides (NOx). The EPA measures NOx at 298 times the toxicity of carbon dioxide (CO2). That’s why PTI has been aggressively testing new road-level solutions as a therapy for mobile-sourced emissions.

Our results are no longer theoretical. We’ve achieved field testing efficacy as high as 60 percent in reducing NOx. Our solutions combine two previously proven technologies: the photocatalytic oxidation of pollutants, using naturally abundant photo-reactive materials combined with field-proven pavement preservation agents, which deliver those materials sustainably into the roadway substructure. We call them “smog-eating roads,” and these therapeutic compounds cost only a fraction of normal road maintenance.

As with other human health conundrums, success is ultimately found in reducing, rather than eliminating, negative impact – a scenario we expect to see played out in the wake of today’s pandemic before long. For now, stay well, and learn more about Smog Eating Roads.

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Here, the purpose is to dig deeper into the chemical compatibility of so-called “bio-based rejuvenators” with asphalt pavements.

Bio-based Product Compatibility Questions

Since their sudden and recent arrival to the pavement preservation market, bio-based products have gained a fair amount of attention yet remain a bit of a mystery to many practitioners. 

There is good reason for them to be reluctant to identify the constituent components of these largely agricultural-derived compounds and to hide behind “trade secret” loopholes when asked to explain exactly how these composites chemically interact with asphalts. 

Asphalt binder has but two molecular components — maltenes and asphaltene. Together, they present a specific, chemically dependent molecular structure that functions in a continuum (chemical matrix) where the maltenes and asphaltene are separately identifiable phases of a singular colloidal solution. Certain of the maltenes provide fluidity and ductility; asphaltene is the filler. Any imbalance or separation of the two embodiments is what causes asphalts to age and fail. This has been “settled science” for nearly 60 years.

A genuine rejuvenator is any compound that restores the proper chemical balance between maltenes and asphaltene. How a compound chemically reacts with asphalt binder is critical — and Maltene Replacement Technology is the only proven method for restoring chemical balance in asphalts.

Agricultural-based products do not contain maltenes. Maltenes are only found in petroleum. They are complex, dense hydrocarbon molecules which naturally complement the chemistry of asphaltene in a maltene-asphaltene matrix, as previously described. 

Agricultural oil derivatives, by contrast, are not molecularly synergistic with multicomplex (polycyclic) petroleums.  They are monocyclic, ultra-light and volatile (unstable), with rapid evaporation rates.  Most are categorized as having varying levels of volatile organic compounds (VOC and SVOC) that are considered damaging to the environment by climate scientists. In addition, these compounds, as solvents, react with asphalts entirely differently than do petroleum maltenes. 

The comparative physiochemical responses at play are equally disparate. They are chemical flux in the case of maltenes and chemical flocculation (floc) with agricultural-derived solvent compounds. Both types may have the initial effect of reducing the viscosity and penetration value of asphalt binder. However, only the maltenes have been proven to do so by promoting improved and restored natural chemical flow within the asphalt binder matrix. 

Agriculturally derived solvents result from the reaction between a vegetable oil and an acid. In contrast with maltenes, a bio-solvent’s chemical structure separates asphalt binder components through dissolution (dissolving), leaving the asphalt binder further depleted of the already depleted hydrating maltenes in aging pavement.  The result is further phase separation in the binder, accelerating the embrittlement of aging asphalts treated with these de facto agricultural-based solvents, i.e., bio-solvents.  In short, bio-solvents act as de-stabilizing reagents to the asphalt binder, which is the opposite of rejuvenation. 

Unlike natural maltene-based rejuvenators, bio-solvents require a drastic chemical conversion (radicalization) of vegetable oil (animal fat) into fatty acid methyl esters (FAMEs) in order to achieve reaction with asphalts. This reaction is chemical dissolution and is specifically engineered to disrupt flux by chemically separating the components of the binder structure, thereby promoting flocculation, which literally means “to flake.” So, bio-solvents are “flocculants” that dry out a compound, dissolving the natural, sustainable fluidity-promoting components by separating and exposing the remaining maltenes in aged asphalts to accelerated oxidation.

In industries such as cosmetics, these solvents commonly are referred to as “drying oils.”  And this is exactly how a bio-solvent interacts with asphalts.  In the computer chip (semiconductor) industry, these compounds are called “flux removers.”  And historically in the oil drilling and paving industries, they are used as asphalt releasers.  They make very efficient industrial-strength cleaners, degreasers and asphalt-release agents. 

The key measurement for chemical flux is time-tested performance, albeit a molecular spectrometer can also be used in the modern age to witness flux. Maltene-rich rejuvenators have been demonstrating their efficacy in over a half a century of exhaustive field applications, including side-but-side comparisons and lab testing.  In contrast, the testing performed on bio-solvents uses an entirely different methodology.

Bio-based Product Testing – “Kb Value”

The key measurement for solvents, including bio-solvents, is Kauri-butanol value (“Kb value”). This test measures the amount of time it takes a given solvent to dissolve the natural rubber from a New Zealand kauri tree. The base solvent is the common sugar-derived alcohol known as butanol. The test is standardized under ASTM D1133-13 ^{1 8}.  The higher a product’s Kb value, the higher its solvency power. ASTM D1133-13 is found on the Safety Data Sheets (SDS) and/or performance specifications of all solvents.  Yet this test appears never to be performed or acknowledged on the bio-solvents marketed as so-called “bio-based rejuvenators.”

All bio-solvents principally come in the form of alcohols and acids.  Several of the bio-based products we’ve seen promoted in the pavement preservation marketplace are simple hydrocarbon-based solvents combined with some “reacted” fatty-acid-based solvent composition; others are nearly pure FAMEs. 

The hydro carbonic solvent in these products typically is d-limonene, a terpene-based solvent in the same family as turpentine, which is derived from pine tree sap.  D-limonene comes from the distillation of citrus fruit rinds. These molecularly identical bio-solvents are very powerful and are often used to dissolve asphalts.

The fatty-acid component (carboxylic solvent) in these products is a methyl ester derived from one of a variety of seeds, beans or grains, such as soybeans, corn, linseed, etc.  For those products using FAMEs as the reacting agent, it should be noted that FAMEs are more commonly referred to as biodiesel fuels, aka bio-fuels. 

Neither hydrocarbon solvents nor the “activated” methyl esters of vegetable oils are flux providers. They are ‘flux removers,” as previously described, and are often labeled as such commercially and sold as industrial-strength solvents.

Here are the Kb values of some common natural and agriculture-derived solvents:

The names or synonyms used on SDS can sometimes be confusing, even misleading. The technical name for soy-derived biodiesel fuel is methyl soyate, methanol being the simplest natural alcohol. Corn-based biofuel is also called corn ethanol. The chemical names commonly used for radicalized linseed oil (flaxseed) are linoleic acid or methyl linoleate.

Agricultural-based bio-solvents are measurably more powerful in terms of solvency than are known petroleum diluents such as kerosene and octane.  All can be used to cut or dissolve petroleum compounds and many are used commercially to that sole purpose. Yet despite a solvent’s mere ability to “soften” asphalts through dissolution (as a high Kb value indicates), no one would suggest using turpentine, kerosene etc. as an “asphalt rejuvenator.” 

Full, factual chemical disclosers on any proposed “asphalt rejuvenator’s” SDS or performance specification would go a long way towards resolving the confusion that currently exists in the asphalt pavement market.

NCAT Warns of Disparate Chemistries

In a recent study by the National Center for Asphalt Technology (NCAT) at Auburn University, NCAT researchers tested several chemical compounds marketed as “rejuvenators” for reactivity with aged asphalt pavements.  NCAT determined that the bio-based products they evaluated chemically softened the asphalt through “lowering the viscosity of the continuous solvent phase” ^{2 9} [of the binder matrix]. Stated simply: the chemical reaction between bio-based products and asphalt exacerbates the separation of maltenes and asphaltene that is already in progress due to oxidation of the aging asphalt pavement. Prominent asphalt chemist Dr. Gayle King refers to this as “diluting the maltene phase.” ^{3 10} Such dilution damages the integrity of the asphalt binder, leading to accelerated pavement aging. 

NCAT further qualified their asphalt rejuvenator study with the following warning:

For optimal restoration of the aged asphalt binder, consideration should be given to the chemical composition of the rejuvenator rather than just its capacity to reduce the viscosity of the aged binder.” ^{4 11}

As NCAT pointed out, bio-solvents simply trigger the process of dissolving the already depleted maltenes left in the aged asphalt binder, rather than replenishing them with the genuine restorative chemistry known as Maltene Replacement Technology.

Ask the Right Question(s)

Identifying the underlying chemicals represented by bio-based products can be as simple as reviewing their SDS, which should provide complete and accurate Chemical Abstracts Service (CAS) number(s) and common names and synonyms for all or most of the product’s constituent components. The simplest way to make certain you’re not introducing a deleterious compound to an asphalt substrate is to request a current and complete SDS and check the Kb value of the substance you’re contemplating using.   In addition, always test new products in the field over multiple periods to ensure asphalt compatibility and performance over time. The proof is in the pavement!

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