What if the best asphalt modifier came from pine needles rather than petroleum? This study compares oils made from three plant wastes and municipal solid waste to see which one actually behaves most like conventional bitumen.

Sustainable road construction is driving research into replacing conventional asphalt binders with bio-based and waste-derived alternatives. Some of these binders have shown potential to reduce greenhouse gas emissions by up to 80%.

The challenge is that alternative binders can alter the chemical and molecular composition of base bitumen, affecting physical characteristics and overall performance. Pyrolysis of biomass and non-recyclable waste is promising because its oils have chemical similarity to fossil fuel-based oils, but performance depends strongly on feedstock type and process parameters.

This study compares pine needles, sawdust, rice straw, and municipal solid waste, adding five percent pyrolyzed oil from each source to VG forty base bitumen. The results point to pine needle oil as the closest match to the base bitumen in molecular structure, thermal stability, rheological response, and aging response.

Alternative binders are not automatically equivalent to conventional bitumen: modifying or replacing bitumen alters its chemical composition. Bio-based materials are typically softer, resulting in lower stiffness than conventional bitumen, so external modifications are often recommended to enhance binder stiffness.

Earlier rheological studies found lower production temperatures and improved low-temperature performance, but compromised high-temperature properties in bio-bitumen. So the sustainability advantage still comes with a compatibility and long-term performance problem that may require molecular-level modification and stabilization.

Pyrolysis oil is produced through thermochemical conversion at temperatures ranging from three hundred to six hundred degrees Celsius, transforming biobased or non-recyclable waste into oil resources. Its chemical structure resembles petroleum-derived counterparts, which is why pyrolysis oil has been investigated as a sustainable binder alternative.

But previous studies reported inconsistent results for oils from different origins, creating a need for systematic comparison across multiple feedstocks. The key issue is source dependence: effectiveness remains highly dependent on feedstock composition, chemical functionality, and molecular characteristics.

The study therefore compares different lignocellulosic feedstocks under identical production conditions to establish source-performance relationships. Most earlier studies focused on a single feedstock source, while this work systematically compares pine needle, rice straw, sawdust, and municipal solid waste.

All four oils were produced under identical pyrolysis conditions and evaluated at the same dosage level, allowing comparison across their different feedstock sources. The comparison combines feedstock characterization with binder-level FTIR, GPC, TGA, storage stability, rheological, and aging assessments to connect feedstock with performance.

The base material is VG forty bitumen, a viscosity-graded binder used here to produce bio-bitumen. Pine needle, sawdust, and rice straw represent the lignocellulosic feedstocks selected for modification. Municipal solid waste is a heterogeneous mixture that can include biodegradable organics, plastics, paper, textiles, glass, and inert materials.

When thermally treated, these materials can yield bitumen-like oxygenated compounds such as phenols, ketones, and acids, making them candidates for bio-binder development. Before pyrolysis, the raw materials were dried to below ten percent moisture, then shredded and crushed to a uniform particle size of two millimeters or less.

The crushed feedstock was heated at one hundred five degrees Celsius for one hour to eliminate residual moisture. A fixed-bed pyrolysis reactor then processed the feedstock at five hundred degrees Celsius for thirty minutes. Identical pyrolysis conditions were applied to every feedstock, allowing differences in oil composition and binder performance to be attributed primarily to feedstock characteristics.

Nitrogen maintained an inert atmosphere at a flow rate of zero point two liters per minute, after which the oil was condensed, separated, and collected. Each modified bitumen replaced five percent of the base binder by weight with pyrolysis oil.

The same dosage was used across all feedstocks to isolate feedstock chemistry from concentration effects. Five percent was also selected as a conservative starting point consistent with the lower bound of reported bio-asphalt dosage ranges. Binders modified with pine needle, rice straw, and sawdust oils are called bio-bitumen, while the binder modified with municipal solid waste oil is called MSW-bitumen.

The oils were physically blended with the base binder using a mechanical stirrer at one hundred thirty-five degrees Celsius for twenty minutes at one thousand revolutions per minute. These relatively mild blending conditions were chosen to preserve the structural integrity of organic fractions in the pyrolysis oils.

Moisture caused foaming during heating and blending, but the foaming subsided and the final mixture became uniform. FTIR spectroscopy characterizes the functional groups and chemical structure of pyrolysis oil and provides a molecular fingerprint of constituents such as phenols, ketones, carboxylic acids, and aldehydes.

By analyzing infrared absorption peaks, FTIR can assess reactivity, aging behavior, and compatibility with bitumen. Here, the spectra were recorded from four thousand to four hundred inverse centimeters at a resolution of four inverse centimeters. The FTIR data were then used to estimate aging index, aromaticity, and moisture sensitivity.

Thermogravimetric analysis identifies volatilization of lightweight compounds, moisture removal, and decomposition kinetics in bitumen. The derivative thermogravimetric curve was analyzed to estimate maximum mass loss and identify peak decomposition temperatures. For this analysis, approximately ten milligrams of binder were tested from thirty to eight hundred degrees Celsius under inert conditions at a heating rate of ten degrees Celsius per minute.

Because phase separation can occur in bio-bitumen and MSW-bitumen, segregation analysis was used to assess storage stability and homogeneity. Samples were conditioned at one hundred forty degrees Celsius for forty-eight plus or minus four hours in aluminum tubes.

They were then cooled to minus four degrees Celsius for four hours, and the top and bottom sections were tested for phase separation. A dynamic shear rheometer measured complex shear modulus and phase angle for all binder samples during frequency-sweep testing. Testing covered ten to eighty degrees Celsius at ten-degree intervals, using different spindle geometries across the temperature range and a fixed strain of zero point one percent.

Data from different temperatures were combined into master curves using the time-temperature superposition principle, with reference temperatures of twenty, forty, and seventy degrees Celsius. The Christensen-Anderson and Marasteanu model generated continuous master curves for complex shear modulus and phase angle, allowing interpolation and extrapolation beyond measured conditions.

Short-term aging used the Rolling Thin Film Oven procedure, with binder exposed to one hundred sixty-three degrees Celsius for eighty-five minutes under heated air. This procedure is designed to simulate volatilization and early oxidative reactions during hot mix asphalt production, storage, and field compaction.

Long-term aging followed in a Pressure Aging Vessel at two point one megapascals and one hundred degrees Celsius for twenty hours. Those conditions simulate gradual hardening that typically occurs over five to ten years of field service. The feedstocks differed clearly in composition, and those differences directly influenced their suitability for bitumen modification.

Pine needle had the lowest ash content at three point eleven percent, relatively low moisture at four point sixty-eight percent, and the highest carbon and hydrogen contents at forty-five point zero one and five point seventy-eight percent. Those characteristics suggest a lower inorganic load and reduced polarity, both beneficial for binder compatibility and aging resistance.

Municipal solid waste had carbon and hydrogen contents comparable to pine needle, but higher ash and sulfur, pointing to a need for further purification. Rice straw and sawdust had higher oxygen contents and the highest ash levels, properties associated with acidity, moisture sensitivity, and accelerated oxidative aging.

Looking across the proximate-analysis rows, volatile mass ranges from sixty-four point zero three percent in SD to seventy-three point eighty percent in PN, while fixed carbon spans fourteen point twenty-one to nineteen point ninety-six percent. In the ultimate-analysis rows, oxygen varies from forty point fifty percent in MSW to forty-five point twenty-two percent in SD, and sulfur appears only in MSW at zero point fifteen percent.

Considering oxygen-to-carbon ratio, hydrogen-to-carbon ratio, ash content, and higher heating value indicators, pine needle emerges as the most promising candidate because of its low polarity, low ash content, and highest energy density. Municipal solid waste is a viable secondary option, although pretreatment to reduce ash and sulfur may be necessary.

Rice straw and sawdust are likely to require additional upgrading to address their high oxygen content and substantial inorganic material. Rice straw oil shows dominant hydroxyl, carbonyl, and carbon-oxygen bands, confirming that it is highly oxygenated and strongly polar. That chemistry often requires pretreatment such as de-ashing, esterification, or mild hydrodeoxygenation to control acidity and moisture sensitivity.

Sawdust has an intermediate chemical character between pine needle and rice straw, with oxygenated functionalities at lower intensities than rice straw and some aromatic features. Municipal solid waste oil has a more hydrocarbon-like signature, but its feedstock heterogeneity means that de-ashing, drying, and attention to halogenated species are needed.

Literature TG-FTIR analyses frequently report hydrogen chloride release from polyvinyl chloride in municipal solid waste, underscoring the need for pretreatment before incorporation into bituminous mixtures. Figure three overlays FTIR spectra for the PN, SD, and RS bio-bitumens, MSW bio-bitumen, and the base binder, highlighting absorbance associated with carbonyl, aromatic, ester and alcohol, sulfoxide, and substituted alkyl or aryl groups.

The modified binders show enhanced features in the twelve-hundred to thirteen-hundred inverse-centimeter region, linked to C–O stretching and S–O stretching, while the figure also marks minor shifts between roughly fourteen-hundred and sixteen-hundred inverse centimeters. These spectral differences matter because they reveal changes in polar functional groups and help distinguish intrinsic carbonyl chemistry from oxidation-related absorption in asphalt.

The bio-bitumen binders show single-step decomposition profiles similar to the base bitumen, while MSW-bitumen shows a two-step curve with a minor mass-loss event around two hundred degrees Celsius. All modified binders had total mass-loss values within plus or minus one percent of the base binder, indicating comparable thermal stability in total decomposition.

The onset temperature for degradation of oxygenated compounds was lowest for pine needle bio-bitumen and highest for the base bitumen. The final decomposition temperature was lower for the base binder and comparatively higher for all bio-bitumen samples, indicating improved thermal stability in the aromatic fraction for the modified binders.

The corresponding peak temperatures span 425.349 degrees Celsius for SD bio-bitumen to 439.23 degrees Celsius for the base binder. The molecular weight distribution analysis found that the base binder had higher number-average and weight-average molecular weights than all bio-bitumen samples.

Among the modified binders, MSW-bitumen had the highest number-average and weight-average molecular weights, suggesting the molecular structure most similar to the base binder. Polydispersity indices ranged from one point fifty-five to one point seventy-four, indicating moderately broad molecular weight distributions.

The relatively low polydispersity values suggest uniformity in binder composition and absence of significant phase separation. In the lower-left panel, the RS bio-bitumen trace is the only curve with four distinct humps, including smaller oscillations during the decline toward the right edge.

By comparison, the other panels show three dominant peaks or broader shoulders, making the RS profile visibly more intricate rather than simply shifted along the elution-volume axis. At twenty degrees Celsius, every modified binder was less stiff than the base binder: pine needle decreased stiffness by twelve percent, rice straw by twenty-nine percent, sawdust by twenty percent, and municipal solid waste by thirty-five percent.

At forty degrees Celsius, the same pattern remained, with pine needle down eleven percent, rice straw twenty-nine percent, sawdust twenty-one percent, and municipal solid waste thirty-nine percent relative to the base bitumen. At seventy degrees Celsius, the decrease was more pronounced: pine needle was down thirteen percent, rice straw thirty-six percent, sawdust twenty-seven percent, and municipal solid waste forty-nine percent.

Overall, the binders became softer and more viscous than the base binder, with municipal solid waste showing the greatest softening and pine needle remaining closest in performance. The tradeoff is potentially better workability and flexibility at the cost of reduced resistance to permanent deformation, particularly at higher service temperatures.

Table 6 reports complex shear modulus and phase angle for five binders at twenty, forty, and seventy degrees Celsius. The complex modulus values range from eight point five times ten to the power of five for BB at twenty degrees to one point seven times ten to the power of five for MSW at forty degrees, while phase angles span forty-five to seventy-eight degrees.

These measurements characterize stiffness and viscoelastic response across representative pavement temperatures, supporting comparisons of temperature- and shear-related deformation behavior. Figure eight compares carbonyl indices in panel a and sulfoxide indices in panel b for unaged, short-term-aged, and long-term-aged binders.

The chart shows increasing carbonyl development with aging across the binder types, while the sulfoxide index follows a similar progression. The authors note that these values are means from three independent FTIR measurements, with generally small standard deviations, supporting the repeatability of the measurements and the reported aging trends.

The sulfoxide index increased progressively with aging severity, while the base binder showed the lowest sulfoxide development under long-term aging. Pine needle bio-bitumen had a lower long-term-aging sulfoxide index than the base binder, suggesting a reduced tendency toward sulfoxidation despite a moderate carbonyl increase.

This indicates a favorable oxidative aging profile, with pine needle showing suppressed polarity increase. Pine needle bio-bitumen showed the most favorable aging behavior among the modified binders, with low sulfoxide formation and a moderate carbonyl increase.

Municipal solid waste bio-bitumen showed moderate oxidation susceptibility, while sawdust showed higher long-term carbonyl and sulfoxide indices. Rice straw had the highest carbonyl and sulfoxide indices and the greatest oxidative hardening tendency at the binder level.

The higher oxidation indices for rice straw and sawdust were consistent with greater susceptibility to aging-related rheological deterioration. Pine needle stands out as the most promising modification for long-term performance, although field-scale embrittlement and cracking still require mixture-level fatigue and low-temperature cracking tests.

Pine needle bio-bitumen demonstrated the most balanced viscoelastic performance, maintaining higher stiffness and lower phase angles closest to the base binder. MSW-bitumen consistently exhibited the lowest stiffness and highest phase angles, especially at elevated temperatures, making it the most deformation-prone binder evaluated.

These rheological trends agree with the FTIR and GPC results, which showed that pine needle oil has the closest chemical affinity to conventional petroleum binders. Figure seven presents master curves for the binders, with complex shear modulus in panels a, c, and e, and phase angle in panels b, d, and f.

The plots use reduced frequency on the horizontal axis and show results at reference temperatures of twenty, forty, and seventy degrees Celsius, respectively. This matters because the curves characterize how binder stiffness and viscoelastic response change across representative pavement temperature regimes.

The feedstock matters as much as the pyrolysis process: pine needle oil produced the most balanced binder, closest to the base bitumen in composition, rheology, thermal behavior, and aging response, though mixture-level testing is still needed.