can I use flexwatt on...

[grammie]

particleboard floor underlayment?? (that's what is written on it) I have a huge shelving unit that would be perfect if I can. tia

[takagari]

im no full expert but if you keep it at the right temps you wont hurt it. ive had it on glass wood. fake wood plastic. never been hot enough to cause an issue

[Dragnbaron]

manufacturers should also be able to give you a 'flash point' on their products. Basically, it's the temperature it takes to catch fire. Anything used for housing has a very high flash point. Plastic has a melting point but even then it takes a nearly 200+ degrees to start melting cheap plastic.
If you are really worried about it you can always get some nice foam insulation (thin or thick type) and set everything on that. It's what i have done in the past and it worked great for me.