Chemistry tells you what is in a compound. Microscopy tells you how it is arranged, which is just as important for properties.
Optical microscopy and dispersion rating
A freshly cut surface is viewed at low magnification, often with reflected light. Undispersed filler shows as bright or dark specks. Several standard approaches rate dispersion on a numeric scale, and some use image analysis to give the percentage of area covered by agglomerates. Poor dispersion lowers tensile strength, tear and fatigue life.
Scanning electron microscopy (SEM)
SEM provides high-magnification surface and fracture images. With energy-dispersive X-ray analysis (EDX) it maps elements, so you can see which particles are silica, which are zinc oxide and which are clay. It is also invaluable for failure analysis: fracture surfaces reveal contamination, voids and filler clusters.
Transmission electron microscopy (TEM)
Ultrathin sections or filler dispersed in solvent are imaged through the sample. TEM gives primary particle size and aggregate structure for carbon blacks and silicas, and it shows how fillers sit in polymer blend phases. Matching measured particle size to the standard ranges identifies the carbon black grade family.
What the images change in a reconstruction
- Black grade. Fine reinforcing blacks and coarse thermal blacks give very different compounds at the same loading.
- Silica type and coupling. Fumed and precipitated silica differ in surface area and structure.
- Blend morphology. Two immiscible polymers form domains of a certain size. The domain size changes damping and wear.
- Mixing quality. A well-dispersed compound and a poorly mixed one can share a recipe and still fail differently.
Limits
Images are local, so many fields and sections must be examined. Sample preparation, such as microtoming a rubber at low temperature, can create artefacts. State the number of fields and the magnification in the report.
