Ultraviolet (UV) light microscope is a powerful tool to visualize the SBS polymeric network in asphalt binders. The microscope makes it possible to gain insight into the polymer and asphalt binder’s compatibility or the blend preparation process’s effectiveness.
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UV microscopy, a polymer-modified asphalt (PMA) binder sample is exposed to UV light, which moves the electrons of the fluorescing portions of asphalt binder to an excited state. These electrons then fall to the ground state, emitting light. The light emitted by the sample travels through the dichromatic mirror and emission filter into the ocular lens or digital camera. The filters allow for separation of fluorescing light from the UV light, making it possible to discern the fluorescing species.
In asphalt binder, only maltenes – specifically aromatics (A) and resins(R) – produce strong fluorescence signals [1]. Coincidentally, SBS blends with asphalt binder by absorbing some aromatics and resins. Therefore, UV microscopy visualizes the specimen’s regions that are SBS rich due to the presence of fluorescing aromatics and resins trapped in the SBS structure. Hence, UV microscopy indirectly visualizes SBS polymers as the polymers themselves do not fluoresce.
There are a few safety hazards associated with UV microscopy. UV light utilized in the UV microscope is harmful to the human eye. The microscope may use a UV light bulb containing hazardous mercury metal. The glass slides are fragile and can break easily, creating sharp objects. Follow the microscope manufacturer’s safety recommendation and wear proper PPE to reduce the risk of injury.
UV microscopy may help predict PMA phase stability. In the following example, PMA was prepared in three distinct asphalt binders. While rheologically similar, each asphalt binder featured a distinct content of aromatics (A) and resins (R). Each PMA contained 4.5 wt%. linear SBS (Kraton™ D1101) and was prepared by one hour of high shear mill blending and six hours of low shear blending at 180°C. UV microscope and polymer separation samples (ASTM D7173) were taken after blending was finished.
The aromatics and resins’ decreasing content is reflected in the UV images by increasing image heterogeneity. The difference between 75.8% A+R and 71.1% A+R is small but distinct. The image corresponding to lower A+R content has lighter spots, indicating relatively higher localized polymer presence. The difference becomes apparent in the 55.2% A+R sample, where distinct image granularity is visible.
In the polymer separation test, a PMA sample is poured into a metal tube and stored at elevated temperature for some time (usually 163°C, 48 hours in the US and 180°C, 72 hours in Europe). If a blend is phase unstable, the polymer will separate from asphalt binder and float to the sample container’s top. After the storage period is over, the sample is cooled to room temperature and separated into three pieces. The top and bottom pieces are evaluated for physical properties such as % recovery or softening point. In this case, the top and bottom samples were subject to MSCR % recovery at 0.1kPa at 76°C (ASTM D7405). The chart below indicates a separation trend similar to that observed in the UV images.
UV microscope is useful in visualizing the PMA’s polymeric network. Caution must be taken during sample preparation, as the UV images’ appearance is highly dependent on the sample preparation technique – more specifically PMA temperature – and whether the sample was agitated or not. Additionally, highly-viscous samples might present preparation challenges, as it may be difficult to extract a drop onto microscope glass without mechanically distorting the drop’s shape.
