It's complicated, and to an extent beyond where I can help given the nature of the Forum.
You can use a scalar to track what material has hit the critical temperature, and you can use an additional scalar to track the age of the first one.
Back into "common knowledge" where I can comment. If a part of your system starts to polymerise does it trigger the whole lot to, or is the initiation temperature still important. The wonders of an old A-level chemistry education.
You are navigating away from the AIS Discovery experience