The research, published in Frontiers in Materials, addressed a persistent gap in asphalt emission science. Most previous studies relied on discrete sampling, capturing a single snapshot of fume composition at one moment in time. Yet asphalt fume emission is inherently dynamic: volatile components surge when heating begins, peak, and then decay as the most readily releasable fractions are depleted. To capture this behavior, the team built an integrated experimental system combining controlled fume generation in a sealed, thermostatically heated flask with mechanical stirring, real-time monitoring of volatile organic compounds and sulfur-containing gases, and offline collection of fumes for detailed chemical analysis by gas chromatography–mass spectrometry.

Repeatability was a central concern. The researchers standardized the thermal history of every sample, heating the asphalt at 140 degrees Celsius for four hours before dividing it into test portions, then reheating each portion for two hours before monitoring. Each test used 500 grams of asphalt, stirred at 300 revolutions per minute, with VOC concentrations recorded every 10 seconds for 90 minutes. Duplicate tests at five temperatures showed relative deviations below 10 percent during the principal response stages, well within the acceptance criterion of 20 percent, confirming that the system could reliably distinguish genuine differences between materials from experimental noise.

The team first compared four penetration-grade 90 asphalt binders from different sources, designated ZH90#, SK90#, JB90#, and GX90#, across temperatures from 120 to 200 degrees Celsius. All four showed VOC responses that increased with temperature, generally peaking before settling into a slowly varying stage. But the magnitude of the response depended strongly on the crude-oil source of the binder. At 200 degrees Celsius, the peak VOC concentration of ZH90# exceeded five times that of GX90#, even though the four binders had similar penetration and softening-point values. The finding carries a practical warning: conventional physical properties alone cannot predict the fume-emission potential of an asphalt binder, and temperature-control strategies should account for the specific source of the material rather than assuming a uniform response across all binders.

The most striking results emerged when crumb rubber entered the picture. The researchers prepared CRMA by incorporating 20 percent by weight of 40-mesh crumb rubber, derived from end-of-life tires with no desulfurization or activation pretreatment, into the ZH90# base binder. At 200 degrees Celsius, the peak VOC concentration of the modified asphalt reached 3,719.1 milligrams per cubic meter, approximately 5.5 times the 672.9 milligrams per cubic meter recorded for the base asphalt alone. The late-stage mean concentration showed a similar ratio of 5.63, and the cumulative response, calculated as the area under the concentration–time curve, was nearly five times higher. The peak also arrived earlier, at 110 seconds rather than 570 seconds, indicating that the rubber-modified binder released its volatile burden more rapidly.

Temperature proved to be the dominant amplifier. Raising the temperature of the modified asphalt from 180 to 200 degrees Celsius increased the peak VOC concentration by 253 percent, the late-stage concentration by 286 percent, and the cumulative response by 275 percent. The researchers attribute this nonlinear surge to three coupled processes: volatilization of light fractions from the base asphalt, enhanced thermal desorption of low-molecular-weight components bound within the rubber matrix, including residual monomers, processing oils, and vulcanization additives, and swelling-induced mass transfer, in which the rubber absorbs aromatic fractions from the binder, expands its interfacial contact, and opens pathways for volatile substances to migrate from the material interior into the gas phase. Notably, the response was not strictly monotonic at every temperature: the modified asphalt showed lower monitored emissions at 160 degrees Celsius than at 140 degrees Celsius, a local anomaly the team interprets as the combined effect of components released before formal monitoring began and temperature-dependent swelling that altered how light components partitioned among the binder, rubber phase, and gas phase.

Sulfur-containing gases added another dimension to the emission profile. The maximum recorded hydrogen sulfide concentration increased from 10 parts per million at 120 degrees Celsius to 31 parts per million at 200 degrees Celsius, while the cumulative H2S response rose from 265 to 1,035 parts per million·minutes over the same interval. Sulfur dioxide responses were consistently higher, with cumulative values increasing from 3,785 to 5,715 parts per million·minutes as temperature climbed from 120 to 200 degrees Celsius. At every temperature tested, the maximum recorded values for both gases occurred at the onset of formal monitoring, followed by an overall decline, suggesting that sulfur species are released most intensely during the earliest stages of sustained heating.

Gas chromatography–mass spectrometry revealed the chemical fingerprints behind these dynamic responses. Fumes from the base asphalt were dominated by petroleum-derived hydrocarbons, including heptane, octane, decane, styrene, naphthalene, and polycyclic aromatic compounds such as phenanthrene and fluoranthene. The modified asphalt fumes retained these aromatic signals but added a distinctive set of rubber-associated compounds: isoprene and its dimer dipentene, which trace to the polyisoprene structure of tire rubber; benzothiazole and methylbenzothiazole, associated with vulcanization accelerator systems; benzothiophene and dibenzothiophene, sulfur-containing heterocycles; carbon disulfide; and tetramethylthiuram disulfide, a typical sulfur-bearing rubber additive. After baseline correction, the cumulative chromatographic response of the modified asphalt fumes was 18 percent higher than that of the base asphalt, with the largest increase, 40.5 percent, concentrated in the 10-to-14-minute retention window. The researchers emphasize that no single compound serves as an independent marker of crumb-rubber origin; the diagnostic value lies in the simultaneous occurrence of this multicomponent fingerprint.

The authors are careful about what their numbers mean. The real-time concentrations are laboratory-specific sensor-equivalent values, determined by the particular reactor scale, gas-flow configuration, and instrumental calibration, intended for comparative assessment among samples and temperatures rather than direct estimation of field emissions or occupational exposure. Laboratory binder tests differ from plant-scale production in processing scale, adsorption by mineral aggregates, ventilation conditions, and thermal history, and field studies have shown that paving temperature, binder content, task, and work rate all significantly affect worker exposure. The findings therefore provide a basis for optimizing processing temperatures and designing targeted fume control, not regulatory limits.

The engineering implications are nonetheless concrete. Because emissions intensified most sharply in the 180-to-200-degree regime, the researchers recommend adopting the lowest temperature that achieves the intended dispersion, swelling, and pavement performance, minimizing high-temperature storage, recirculating heating, and repeated reheating, and using multipoint temperature measurements to prevent localized overheating near 200 degrees Celsius. Fume capture should cover the entire processing sequence, from crumb-rubber addition through mixing, swelling, transfer, and storage, with local exhaust ventilation activated before processing begins, since the highest sulfur-gas responses occurred at the onset of monitoring. Collected fumes should be treated in stages, removing particulates and condensable droplets first, then applying VOC adsorption, condensation, or oxidation according to composition, with dedicated units for H2S and SO2. And because the unresolved VOC maximum at 120 degrees Celsius showed that even lower-temperature operation does not eliminate prolonged fume release, ventilation and capture must remain active throughout both heating and holding. For a recycling technology poised to expand worldwide, the study delivers a clear message: the greenest binder is only as clean as the temperature at which it is processed.

Subject of Research: Dynamic fume emission behavior and chemical composition of crumb-rubber-modified asphalt compared with base asphalt across processing temperatures

Article Title: Characteristics of fume components and dynamic emission behavior of crumb-rubber-modified asphalt

Article References: Luo, L., Dai, J., Zhu, W., Li, C., Yu, L., Liu, Y., Li, B., & Zhang, Z. (2026). Characteristics of fume components and dynamic emission behavior of crumb-rubber-modified asphalt. Frontiers in Materials, 13, Article 1979048. https://doi.org/10.3389/fmats.2026.1979048

Image Credits: AI Generated

DOI: 10.3389/fmats.2026.1979048

Keywords: crumb-rubber-modified asphalt, asphalt fumes, volatile organic compounds, hydrogen sulfide, sulfur dioxide, gas chromatography–mass spectrometry, tire recycling, thermal desorption, emission control, pavement materials, Frontiers in Materials, dynamic emission behavior

The core problem is chemistry. Rubber is a cross-linked network of natural and synthetic polymers filled with carbon black, plasticizers, and vulcanizing agents, and its surface is chemically inert. Asphalt, by contrast, is a complex colloid of asphaltenes, resins, and light oily fractions. When the two are simply stirred together, the mismatch in their physicochemical properties produces weak interfacial bonding and poor compatibility, leading to phase separation during storage. Paradoxically, rubber powder also swells by absorbing the lighter components of asphalt, which increases particle volume and interparticle friction, building a network that drives viscosity skyward. That high viscosity is a double-edged sword: it signals good deformation resistance, but it forces mixing and compaction at higher temperatures, wasting energy and releasing harmful fumes.

Researchers have attacked this problem with physical, chemical, and biological activation of the rubber surface, and microwave treatment has emerged as a favorite because it is cheap, fast, and efficient. Microwaves generate heat inside the rubber particles themselves, breaking sulfur cross-links and creating reactive surface sites. Bio-oil pretreatment offers a complementary route: aromatic-rich oils swell the rubber and introduce low-molecular-weight constituents that soften the interface. The new study, led by Xiaowei Feng, Chaocai Qin, and colleagues, goes further by combining the two approaches and then adding polyphosphoric acid, or PPA, a viscous inorganic acid that costs roughly half as much as the widely used SBS polymer modifier and is prized for improving storage stability and high-temperature deformation resistance.

The experimental design was exhaustive. The team ground waste truck tires into 40-mesh powder and tested three bio-oils as pre-swelling agents: castor oil, a bio-asphalt derived from biomass, and waste cooking oil from fried soybeans. Rubber powder was oven-dried, mixed with bio-oil at oil-to-rubber mass ratios of 0.3, 0.5, and 0.7, and left to soak for twelve hours at room temperature so the oil could fully penetrate the particles. The swollen powder then went into a household-grade microwave oven operating at 2450 megahertz and 400 watts for irradiation times ranging from two to five minutes. Preliminary gradient tests revealed sharp limits: below two minutes, insufficient energy input left the rubber under-devulcanized; beyond four minutes, excessive thermal input triggered oxidative chain scission and thermal cracking of the bio-asphalt, complete with smoke and sparks. In total, 26 activation formulations were prepared and blended into asphalt at 20 percent rubber content using high-shear mixing at 4000 revolutions per minute.

The macroscopic test results painted a nuanced picture. Penetration values of the activated rubber asphalt rose by 10 to 270 percent relative to ordinary rubber asphalt, with waste cooking oil producing the softest binders and bio-asphalt the stiffest, because the added light fractions diluted the asphaltene content and reduced viscosity. Softening points told a different story: castor oil and waste cooking oil treatments cut high-temperature deformation resistance by 14.7 to 28.4 percent, while bio-asphalt activation actually raised the softening point above that of conventional rubber asphalt in most cases, peaking at four minutes of microwave exposure. Ductility, a proxy for low-temperature performance, improved dramatically with waste cooking oil, reaching 13.80 centimeters, a 92.5 percent gain over the control, while castor oil treatments reduced ductility by as much as 52.4 percent because swelling without dissolution concentrates stress during stretching.

Viscosity and storage stability proved to be the decisive criteria. Nearly all activated formulations showed reduced viscous resistance at 135 degrees Celsius, with castor oil and waste cooking oil cutting viscosity by 49.1 to 85.6 percent, easing mixing and compaction temperatures substantially. Under the Chinese technical standard for rubber asphalt pavements, which requires viscosity below 3 pascal-seconds at 135 degrees Celsius, almost every formulation qualified. Storage stability, measured by the softening point difference after 48 hours at 163 degrees Celsius, improved by roughly half for the best waste cooking oil formulations, with values of 2.8 and 2.7 degrees Celsius representing reductions of 50 and 51.7 percent compared with conventional rubber asphalt. The mechanism, the authors suggest, is that light bio-oils help the rubber swell more fully while grafted biomolecules increase surface polarity and strengthen the bond to the asphalt phase.

To select the single best recipe from the crowded field, the team applied grey relational analysis, a multi-criteria optimization method that scores each formulation against an ideal reference across penetration, softening point, ductility, elastic recovery, viscosity, and segregation. The winner was unambiguous: waste cooking oil at an oil-to-rubber ratio of 0.3 with four minutes of microwave irradiation, designated W0.3 + 4, achieved the highest correlation degree of 0.8748. This formulation delivered the best combination of storage stability and construction workability, and it became the foundation for the next phase of the work, in which polyphosphoric acid was added at 1.2, 1.7, and 2.2 percent by mass of base asphalt to recover the high-temperature performance lost to the oily light fractions.

The PPA results were striking. Adding the acid lowered penetration and raised the softening point in proportion to dosage, and the 2.2 percent formulation reached a softening point of 63.2 degrees Celsius, actually exceeding conventional rubber asphalt by 1.3 degrees. Viscosity at 135 degrees Celsius increased by 43.5, 82.0, and 139.7 percent for the three dosages respectively, yet remained below that of ordinary rubber asphalt for dosages up to 1.7 percent, preserving workability. Storage stability improved further: the segregation index fell from 35.85 percent for conventional rubber asphalt to 25.92 percent for activated rubber asphalt, and dropped again with increasing PPA, with softening point differences of 2.0 and 1.5 degrees Celsius at 1.7 and 2.2 percent dosages, comfortably within the 2.5-degree threshold that Chinese specifications classify as exceptional phase stability.

Microscopic analysis explained why the strategy works. Scanning electron microscopy showed that untreated rubber powder has a dense, smooth surface with limited pore connectivity, which hinders bonding and allows particles to slip and segregate. After oil pre-swelling and microwave treatment, the surface loosened, pores multiplied, and a continuous biomolecular film formed, rich in amide functionalities that raise interfacial polarity. Fourier transform infrared spectroscopy confirmed real chemistry: the disappearance of the 1540 per centimeter peak indicated microwave-induced desulfurization, while new ester carbonyl and sulfone peaks showed that waste cooking oil molecules had grafted onto the rubber and oxidized during processing. In the PPA-modified binders, new hydroxyl and phosphorus-oxygen peaks revealed esterification reactions that the authors describe as chemical anchoring, with phosphorus-containing products acting as in-situ surfactants at the rubber-asphalt interface.

Gel permeation chromatography added the final molecular-level insight. Microwave desulfurization reduced the weight-average molecular weight of the binder from 2774 for conventional rubber asphalt to 2199 for the activated version, reflecting degradation of large rubber chains. PPA reversed the trend, raising the weight-average molecular weight to 2809 at 2.2 percent dosage and increasing the large molecular size fraction, evidence that the acid links polymer chains into a network that resists shear deformation at high temperature. Taken together, the findings outline a practical, low-cost route to sustainable pavements: kitchen waste oil and a microwave to make scrap tire rubber compatible with asphalt, and a modest dose of an inexpensive acid to give the resulting binder the backbone to survive the summer heat. For the billions of tires discarded annually, that could mean a far more useful afterlife beneath the wheels of the vehicles that produced them.

Subject of Research: Composite activation of crumb rubber with bio-oil and microwave radiation to improve the performance and storage stability of rubber-modified asphalt

Article Title: Preparation of bio-oil-microwave composite activated rubber powder and research on its asphalt properties

Article References: Feng, X., Qin, C., Ning, Z., Zhang, H., Yang, Y., Yang, X., & Liang, J. (2026). Preparation of bio-oil-microwave composite activated rubber powder and research on its asphalt properties. Case Studies in Construction Materials, 25, Article e06418. https://doi.org/10.1016/j.cscm.2026.e06418

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06418

Keywords: crumb rubber asphalt, waste cooking oil, microwave activation, polyphosphoric acid, storage stability, asphalt modification, tire recycling, bio-oil, rheology, sustainable pavement, gel permeation chromatography, FTIR analysis