Objective: For pocket-sized rice hand warmers, what kind of thread should I use?

Which of the following solutions is more sound in terms of naive physics reasoning?

Options are:
[a]. The thread must be 100% cotton so that you can heat it in the microwave. Polyester and other synthetics can burn or melt.
[b]. The thread must be 100% polyester so that you can heat it in the microwave. Cotton and other synthetics can burn or melt.
[a].