Analytical chemistry is powerful, but a cured rubber is the end of a chain of chemical reactions. Several things that matter are not recoverable, and any reputable report says so.
1. The exact curatives
Peroxides decompose during cure. Coagents react into the network. Accelerators and activators turn into fragments and salts. What remains is a trace, or nothing. A lab can often say "peroxide-cured" or "sulfenamide accelerator present", but not which peroxide, how much, or which coagent.
2. The supplier's grade
Two fluoroelastomers with the same monomers and similar fluorine content may come from different makers and behave a little differently. Analysis identifies the type, perhaps the comonomer ratio, not the product code on a datasheet. The same holds for HNBR: acrylonitrile content and residual double bonds can be estimated, not the trade name.
3. The mixing and processing history
Mixing order, temperature, shear, ageing of the mixed stock and cure conditions are not written in the finished part. They shape dispersion, crosslink density and compression set. See the insights section for what mixing changes.
4. Trace and proprietary additives
Processing aids, tackifying resins, release agents and specialty modifiers used at fractions of a part per hundred may be invisible to standard methods.
5. The cure state and the history of the part
How fully was it cured? Was it post-cured? How much has it aged since? Swelling tests and thermal analysis give hints, but a used part has a story the lab can't fully read.
What to do about it
- Treat the reconstruction as a candidate. Mix it, cure it, test it and compare.
- Spend the effort on the physical profile: hardness, modulus, tensile, elongation, compression set at the right temperature, fluid swell and low-temperature tests.
- Use the cure curve (rheometer) of the trial compound and compare it with a similar part's cure behaviour.
- Accept that two compounds can be different inside and equivalent in service. That is the real objective: matching performance, not copying chemistry.
