ASPHALT ROAD BIO-BASED REJUVENATORS – FUNCTIONALITY REVIEW

In recent years, there has been no shortage of confusion (and some intentional misdirection) in the asphalt pavement preservation marketplace in relation to the definition and understanding of maltenes and the role they play in rehabilitating and extending the life of asphalt roadways. For a clear understanding of the natural, resilient chemistry Pavement Technology calls Maltene Replacement Technology, and its demonstrative benefits, especially in comparison with alternative methodologies for preserving asphalt pavements, we refer you to The Science of Maltene Replacement Technology for Sustainable Asphalts.

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 1}.  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 2} [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 3} 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 2}

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!

1. Active Standard ASTM D1133-13: Standard Test Method for Kauri-Butanol Value of Hydrocarbon Solvents, ASTM International, Subcommittee: D01.35, Book of Standards Volume: 06.04.
2. Asphalt Technology News, Spring 2019, Volume 31, Number 1, National Center for Asphalt Technology (NCAT) at Auburn University.
3. Pavement Technology, Inc. v. Biobased Spray Systems, LLC, United States District Court Northern District of Ohio, Case No. 1:18-CV-01698-DAP, filed July 23, 2018.