The goal of these pages is to explain the naturally resilient chemistry Pavement Technology refers to as “maltene-replacement technology,” and its demonstrative benefits in comparison with alternative methodologies for preserving asphalt pavements. Use the links below to delve more deeply into each concept discussed or use the form on this page to request a comprehensive white paper on this topic.
Sustainable Asphalt Pavements: The Science Behind Maltene-Replacement Technology
In recent years, there has been no shortage of confusion (and some intentional misdirection) in the asphalt pavement preservation marketplace regarding the definition and understanding of maltenes and the role they play in rehabilitating and extending the life of asphalt roadways. Let’s clear up the confusion.
As defined by the U.S. government’s “Definition and World Resources of Natural Bitumens1 , maltenes are the n-alkane- (that is, pentane or heptane)-soluble molecular components of asphalt. There are two molecular structures common to all asphalts, regardless of their oil field source. Maltenes are basically the “glue” that binds asphalt pavements together, imparting flexibility, fluidity, and adhesion properties; asphaltenes are largely responsible for an asphalt pavement’s rigidity. Collectively, these properties are the key to an asphalt pavement’s ability to withstand considerable environmental and traffic stresses.
- As the more fluid resinous and oily fractions found in asphalt binder (aka asphalt cement), maltenes are characterized by a lower molecular weight than asphaltenes, are highly soluble, and chemically reactive (volatile).
- Asphaltenes are the more viscous substrate in asphalt. They are characterized by a heavier molecular weight, are not easily soluble, and are less chemically reactive.
Maltenes can be further broken-down using adsorption chromatography in the presence of an acid reagent into four distinct subfractions:
- Polar Compounds2
(aka nitrogen bases)
- highly reactive petroleum resins that act as a colloidal dispersion stabilizer or “peptizer” for the asphaltene substrate, imparting distinct and necessary characteristics to asphalt pavements
- First Acidaffins (A1)
- aromatic resinous petro-hydrocarbons, with or without O, N and S, acting as a chemically compatible dispersion agent for the peptized asphaltene
- Second Acidaffins (A2)
- straight chain or cyclic unsaturated petro-hydrocarbons (olefins) that are somewhat oily and resinous, imparting distinct and necessary characteristics to asphalt pavements
- Paraffins3 (aka Saturates)
- straight or branch-chain saturated hydrocarbons (aka naphthene-aromatics) that highly oily and impart distinct and necessary characteristics to asphalt pavements
In 1959, Dr. Fritz Rostler, working in America’s petroleum and rubber industry with his associate Richard M. White, isolated and tested the chemical reactivity of asphalt’s two major components, asphaltenes and maltenes, in an effort to identify what combination of these two molecular structures would most likely produce resilient thermoplastic performance.
By using sulfuric acid to separate the soluble components (highly volatile and reactive maltenes), and then using an n-pentane solvent to separate the insoluble components (heavier, more stable asphaltenes), they observed the asphaltenes in asphalt, when exposed to lab-simulated sun and weather, increased over time, while the asphalt’s maltene content diminished. This suggested a continuously interactive molecular structure that would be receptive to maltene replacement treatments penetrating the asphalt matrix.
Once component separation was complete, Rostler et al. conducted abrasion-resistance (pre-weathering and post-weathering) and cohesion testing, which definitively demonstrated the loss of the low-molecular-weight maltene components in naturally stressed asphalt is largely responsible for the cracking and hardening seen in aging pavement. Subsequent analysis determined some maltene content is initially lost during the high-temperature asphalt production process, and more is depleted when the highly volatile maltenes in asphalt pavements are exposed to natural oxidation. The question then arose:
If the maltene content in asphalt pavements could be replaced, could the pavement’s flexibility and adhesion properties be restored?
Thus, genuine asphalt rejuvenation in the form of maltene-replacement technology was born, at least in theory. It was the Witco Corporation, the original manufacturer of today’s Reclamite asphalt rejuvenation technology, that undertook the research necessary to identify the most effective method for feeding maltene-rich chemistry to new and aging pavements.
Through their break-through examination of the chemical reactivity of an asphalt binder’s sub-components, Rostler et al developed ASTM Test D-2006-704 , which accurately identifies the relationships between the various maltene fractions with the goal of determining the optimal maltene content distribution within an asphalt rejuvenating agent. They did so by rank-ordering maltene fractions by their changeability or vulnerability to volatilization and oxidation. The idea being the more vulnerable fractions are lost first and most often and thus should be the first and most often replaced by maltene-replacement technology.
This exercise, commonly known as the Rostler Analysis, is the measure still used today to determine the efficacy of a genuine asphalt rejuvenation treatment.
During the first half of the 20th Century, in the modern era of civil engineering, surface treatments such as coal tar were introduced in the hope of extending the life of existing asphalt pavements. It was soon discovered that coal tar is chemically incompatible with asphalt and therefore incapable of bonding with the molecular structure of the asphalt binder. As a topcoat, coal tar can temporarily enhance the appearance of the underlying aging, gray asphalt pavement, but it wears off quickly and cannot be considered a rejuvenating agent.
Asphalt-based emulsions and fogs provide some surface-based protection by shielding the underlying asphalt pavement from direct and continued exposure to the elements. However, since any maltene content within these chemistries is already bonded to the material’s asphaltene content when it is applied to the road, emulsions and fogs are incapable of actively bonding with its molecular structure, and therefore are incapable of genuine rejuvenation.
Maltenes, by definition, are petroleum-based. Chemistries derived from oranges, soy beans, corn or other so-called “natural” ingredients do not contain maltenes and are therefore incapable of replacing them. The Rostler Analysis of the chemical reactivity of a material’s asphaltene and maltene components cannot be used to measure the efficacy of bio-based chemistries because they have neither asphaltenes nor maltenes to fractionalize.
The confusion in the market over bio-based “rejuvenators” can be most directly attributed to the replacement of the Rostler Analysis (ASTM Test D-2006-70) with a new standard: ASTM D2007 SARA. ASTM D2007 SARA is highly useful as a separation technique; as such, it is applicable to a wide range of industries.
The new test fractionalizes the components of tested materials, but instead of analyzing the chemical reactivity of a binder’s components, it is essentially a test that measures the solubility of a tested material’s components without differentiating the chemical reactivity of distinct fractions.
ASTM D2007 SARA is looking for proper suspension between an asphalt’s heavier asphaltenes and its lighter components as an indication of the tested material’s fluidity and workability. As such, it is good for identifying the stability of an asphalt emulsion or an extender oil compound. However, it is useless for determining a material’s ability to rejuvenate an asphalt pavement.
Most significantly, ASTM D2007 SARA may produce false positive results on bio-based products and other low-molecular-weight non-petroleum composites, including materials that are asphalt-damaging solvents. Powerful bio-solvents may appear to soften treated asphalts when, due to their high Kauri-butanol (Kb) value5 , they are actually chemically separating the asphalt matrix. If you wish to avoid using powerful solvents on your roadways, always inquire into the Kb value of the materials you are considering.
Historically, such materials were marketed as asphalt releaser agents because they promote dissolution of a petroleum suspension phase such as maltenes. In essence, reagents destroy asphalt pavement through chemically-induced separation of the asphalt binder’s components. Once the asphalt matrix has been compromised, future maltene-replacing rejuvenation will likely be unfeasible.
Although marketed as eco-friendly, some bio-based reagents, despite their “natural” derivations, are substantially composed of ecologically damaging volatile organic compounds (VOCs), which climate scientists believe to be ozone precursors toxic to humans. These types of concerns require further research to resolve.
The research is still out on bio-based materials. Raising specific questions about the chemical properties of any product you are considering is a good precaution to take before applying bio-based materials to your roadways.
In our work in the field, Pavement Technology is increasingly observing accelerated aging characteristics in newly built asphalt pavements. Societal pressures to use recycling asphalts and bio-based additives on America’s roadways may be contributing to this problem.
Specifically, in an effort to produce high temperature strength (reduced rutting risk) and low temperature fluidity (reduced cracking risk) during asphalt mixing, some mix additives and recycling agents appear to be producing detrimental results in the field. It may be that material additives, which are essentially high-Kb-value solvents, when subjected to high-temperature laboratory testing achieve the sought-after rigidity in asphalts by dissolving the maltene-phase content, while the bio-solvent itself is volatized by the increased heat. Conversely, when subjected to low-temperature laboratory testing, the maltenes may be chemically separating while the bio-solvent is suspended in an aqueous phase, thereby providing a false positive indicator of reduced viscosity.
To be sure, more research is needed, but inquiring into the Kb value of the materials you are considering for your roadways is a reasonable first-step precaution to take when asphalt rejuvenation is your goal.
Ultimately, the proof is in the pavement, and real-world testing6 further supports the results of Dr. Rostler’s analyses. For nearly 50 years, maltene-replacement technology has been used on asphalt roadways across America. During that time, a wide number of side-by-side (treated vs. untreated) pavement comparison studies have demonstrated the effectiveness of maltene replacement. Such true rejuvenators, which return molecularly exact, depleted chemicals into the asphalt matrix, extend roadway life when new pavements are treated within their first two years of service, then retreated three to five years later.
The initial treatment immediately restores those maltenes lost during the high temperatures used during asphalt mixing. The subsequent retreatments restore those maltenes depleted during exposure to sunlight and weather. Depending upon road conditions, additional retreatments are sometimes recommended, with the goal of achieving a multi-decade service life before pavement rebuild is required. Roadway longevity is the ultimate test of a sustainable solution, and maltene-replacement technology is both lab-tested and field-proven to extend the life of asphalt pavements.
- Vladimir Kalichevsky and Stewart C. Fulton, 1931 patent US1926523A
- Heavy Oil Science Center, More About the Chemistry of Asphaltenes and Maltenes, Foster Learning Inc. (http://www.lloydminsterheavyoil.com/asphaltenes2.htm)