Pol-y-es-ter......... Really?!...... Are you sure??..... I would have never thought!...... Giggle
I was taught by an incredibly superb UK quilt teacher, Barbara Chainey, that you can test all threads, not matter what they are made of. You take a length, about 16 inches, wind the ends around your fingers on both hands and .... pull! You'll find that some threads tear immediately and most likely are too weak for any seams where they have to cope with a certain amount of pressure, others are impossible to break that way (They are the ones that may cut and shred your fabric and seams), and those that have some strength, but then break. Those are the most suitable for piecing, for example. I always thought that was excellent advice.