Analysis of the Mechanism and Effectiveness of Lignin in Improving the High-Temperature Thermal Stability of Asphalt

The use of lignin, which is a by-product of the pulp and paper industry, in the development of asphalt binders would contribute to waste reduction, providing environmental, economic, and social benefits. In this study, samples of lignin-modified asphalt binder samples with different content of lignin (3%, 6%, 9%, 12%, and 15%) and unmodified asphalt (control) were tested using Fourier transform infrared spectroscopy (FTIR), dynamic shear rheometer (DSR), and thermogravimetry. The mechanism and effectiveness of lignin in improving the thermal stability of asphalt at high temperatures were analyzed. The FTIR analysis shows that no new characteristic absorption peak is seen in the infrared spectral of the lignin-modified asphalt binder samples, and some bands characteristic of lignin-related peaks gradually increased with the increase of lignin content. This suggests that the modification of lignin-modified asphalt binder samples was due to physical blending rather than chemical modification. The increase of lignin content in the lignin-modified asphalt samples increases the complex shear modulus G* of the samples and decreases the phase angles of the samples. Similarly, the anti-rutting performance (G*/sinδ)of the samples improves with the increase in lignin content, but this is not significant after any addition of lignin that exceeds 12% of asphalt mass. Thermal characterizations show that the thermal decomposition rate of lignin is lower, and its residual amount is higher compared to that of asphalt, which is a major reason for the improved stability of lignin-modified asphalt binders at high temperatures. The effect of lignin on the thermal stability of asphalt is dependent on both lignin content and temperature. It has a positive effect on the thermal stability of asphalt at high temperatures within the range of asphalt service temperature (25°C–200°C). Additionally, from the pyrolysis viewpoint, it was explained that excessive lignin addition is not beneficial to the thermal stability of asphalt at high temperatures, which is consistent with the DSR test result conducted high temperatures.