Once the ash of a rubber compound is dissolved in acid, the question becomes: how much of each metal is there? Two techniques dominate.
Atomic absorption spectroscopy (AAS)
A solution is sprayed into a flame or heated graphite tube. Atoms of a specific metal absorb light of a specific wavelength from a lamp, and the loss of light is proportional to concentration.
- One element at a time, with a different lamp for each.
- Very reliable and low cost for a short list of metals such as zinc, calcium and magnesium.
- Needs a dilution series of standards and careful matrix matching.
Inductively coupled plasma (ICP)
A plasma at several thousand degrees excites atoms in the solution, which emit light at characteristic wavelengths. Many elements are read at once.
- Fast, multi-element, wide dynamic range, and good for unknown samples.
- Spectral interference between elements has to be managed.
- Mass spectrometry versions (ICP-MS) reach very low levels, useful for trace contaminants and heavy metals.
From metal to ingredient
Convert each metal result to the oxide or compound the compounder most likely used.
- Weigh the ash and note the fraction dissolved.
- Measure the concentration of the metal in solution and scale back to the original sample mass.
- Convert to the oxide, using atomic masses. For zinc oxide, divide metal mass by about 0.803 to get ZnO mass. For magnesium oxide, divide by about 0.603.
- Express as phr against the polymer fraction.
A worked example is in From weight percent to phr.
Useful cross-checks
- The sum of oxides plus insoluble residue should approach the total ash weight. A big gap means something was lost, such as carbon dioxide from carbonates.
- Zinc above what a normal cure aid would need (about 3 to 5 phr in many sulfur recipes) hints at a zinc-based filler or zinc-rich accelerator system.
- In fluoroelastomers, calcium and magnesium in the expected ratio point to the usual acid acceptor pair.
Lab practice
Use certified standards and blanks, report detection limits, and always state how the sample was ashed and dissolved. Differences in digestion are a common reason for two labs disagreeing.
