A vulcanised rubber does not dissolve. That is the point of vulcanisation. To identify the polymer you need to break the network into small molecules first, and pyrolysis does exactly that.
The principle
Heat a small piece, typically a fraction of a milligram to a few milligrams, rapidly to a high temperature such as 500 to 800 °C in an inert atmosphere. The polymer chains break down into characteristic fragments. Those fragments are then identified by infrared spectroscopy of the condensed pyrolysate or by separating and identifying them in a gas chromatograph with a mass spectrometer (Py-GC-MS).
What fragments say
- Natural and synthetic polyisoprene give isoprene and dipentene.
- Polybutadiene gives butadiene and 4-vinylcyclohexene.
- SBR gives butadiene, styrene and related products in a ratio that points to the styrene content.
- Nitrile rubber gives acrylonitrile alongside butadiene products, and the ratio hints at the acrylonitrile content.
- EPDM gives short hydrocarbon fragments, and the diene shows as small peaks specific to the termonomer.
- Silicones give cyclic siloxanes such as hexamethylcyclotrisiloxane.
- Fluoropolymers give fluorinated alkenes and, at high temperature, hydrogen fluoride.
Strengths
- Works on cured, black-filled rubber, where transmission IR struggles.
- Needs very little sample, so it suits small seals.
- Gives information about comonomer ratio, not just polymer family, when run quantitatively.
Limits
- Pyrolysis fragments of different polymers can overlap, especially in blends.
- Calibration against known standards is needed for any ratio.
- Fluoropolymers and chlorinated polymers release corrosive gases such as HF and HCl. The pyrolyser, the lines and the column need to be suited to them, and the lab needs a safety plan.
- Heavy loading with some fillers or additives can influence the pyrolysis pattern.
Where it fits in a programme
Pyrolysis is the partner of infrared spectroscopy. When two completely independent methods give the same polymer, you can report it confidently. When they disagree, the sample may be a blend or a polymer you did not consider, and that is worth finding out before you mix a single trial batch.
Sources and further reading
- ASTM D3452, rubber: identification by pyrolysis-gas chromatography.
