The final step of reverse engineering is the only one that proves anything. You mix the proposed recipe, cure it and put it next to the original.
Prepare the comparison
List the properties that matter for the duty and agree the test conditions up front. For a seal compound this normally includes:
- Hardness (Shore A) and density
- Tensile strength, elongation and modulus at 100 %
- Compression set at the service temperature, not just room temperature
- Fluid swell in the real fluid at the real temperature
- Low-temperature behaviour (TR10, or brittleness) where the duty is cold
- Cure curve on a rheometer (ML, MH, ts2, t90)
Include the original, the trial compound, and whatever the customer's specification says.
A trial in four moves
- Mix small. A laboratory internal mixer or a two-roll mill is enough for a first pass. Keep the mixing sequence consistent and record temperatures.
- Check the cure curve first. If the rheometer curve is very different, stop and fix the cure system before testing anything else.
- Compare the physical profile. Cure to optimum, post-cure if the polymer needs it, and run the property list.
- Adjust one thing at a time. Hardness off? Adjust filler. Modulus low? Adjust crosslink density or filler structure. Set high? Look at the cure and post-cure and at the filler type.
How close is close enough
There is no universal tolerance. A useful starting point:
| Property | Reasonable match |
|---|---|
| Hardness | within ±3 points |
| Tensile and modulus | within about 10 % |
| Elongation | within about 15 % |
| Compression set | no worse than the original, ideally better |
| Fluid swell | within the original's range |
| TR10 / low-temperature | no worse than the original |
Anything tighter should be justified by the application. A specification that says "match the original" without numbers needs to be turned into numbers.
How many iterations
Plan for two to four rounds for a straightforward compound and more for compounds in demanding duty such as sour gas, steam or rapid gas decompression. The final proof is the application test, not the lab table.
Document what you did
Keep the recipe, the mixing record, the cure curve and the test results together. That file is what turns a one-off sample into a repeatable compound.
