基于热分析与低温光谱的沥青热可逆老化机理

为了研究沥青胶结料在恒定低温储存过程中产生的热可逆老化机理,运用调制式差示扫描量热分析(MDSC)和低温红外光谱(LT-FTIR)技术对沥青胶结料热信号与红外光谱的热历史依赖性进行了试验分析,以确定2种不同分子量的线性饱和链烃(C₂₀H₄₂和C₃₂H₆₆)对热可逆老化的贡献。研究结果表明:沥青恒定低温硬化并非所有沥青的固有特性,存在不受恒定低温热历史影响的沥青胶结料;低分子量的饱和长链烷烃(石蜡)因其与沥青具有较好的相容性,会通过扩散的方式随着恒定低温时间的延长逐渐从沥青基体中析出,从而导致沥青的恒温硬化现象;由于高分子量的饱和长链烷烃与研究采用的沥青在相同条件下相容性差,其石蜡与沥青的二相分离结构并没有随着恒定低温时间的延长产生明显的变化。与热分析相比,低温红外光谱技术可在较低的降温速率下直接用于测试固态沥青中的石蜡分子单元而不会产生热滞后效应。石蜡的结晶会导致红外光谱在735~715 cm^-1处形成吸收峰,且吸收峰信号随温度降低而增强。沥青中的石蜡初期(0~8 h)析出较快,随着时间延长,析出速率放缓,持续时间可长达72 h。通过试验的直接观测,确定了沥青中低分子量石蜡的持续析出或沉淀是导致所用胶结料热可逆老化的根本原因。

Mechanisms of Thermoreversible Aging of Asphalt Binders Based on Thermal Analysis and Low-temperature Fourier-transform Infrared Spectroscopy

The thermal signals and spectra of two asphalt binders with different degrees of thermoreversible aging were investigated using modulated differential scanning calorimetry and low-temperature Fourier-transform infrared spectroscopy. Two linear saturated chain hydrocarbons with different molecular weights (C₂₀H₄₂ and C₃₂H₆₆) were added to the asphalt binder to determine the contribution of the molecular weight of solid paraffin to thermoreversible aging. The results show that thermoreversible aging is not an inherent property of all asphalt binders, and some asphalt binders are not affected by thermoreversible aging. Low-molecular-weight saturated long-chain alkanes (paraffin) have good compatibility with the base asphalt. With the extension of cold storage at low temperatures, the low-molecular-weight saturated long-chain alkanes gradually precipitate out from the asphalt matrix through diffusion, leading to the isothermal hardening of the asphalt binder. High-molecular-weight saturated long-chain alkanes have poor compatibility with asphalt. Hence, the presence of paraffin and asphalt in a two-phase separation structure does not change significantly with time. Compared to thermal analysis, the low-temperature infrared spectroscopy technique can directly determine the paraffin molecular units in a solid state at a lower cooling rate without thermal hysteresis. The crystallization of paraffin results in the formation of absorption peaks at 735-715 cm^-1 in the infrared spectrum, and the absorption peak signal increases with decreasing temperature. The paraffin in the asphalt binder precipitates rapidly at the initial stage (0-8 h). As time progresses, the precipitation rate declines, and the duration can reach 72 h. Through direct observation of the experiment, it is evident that the continuous precipitation of low-molecular-weight paraffin in the asphalt is the primary cause of the thermoreversible aging of asphalt binders investigated in this study.