Why can't some plastic bags with the microwave

Why can't some plastic bags with the microwave? One, the microwave heating principle is the principle of the microwave food, use food is contained in strongly polar water molecules as a medium, can absorb energy from the microwave, violent vibration, friction in the molecule, which generate heat. Used the microwave users know, metal or metal containers can't be put in the microwave. Because the metal is made up of strong polar molecules. Microwave belong to physical heating, cooking food in much the same way like cooking with open flames, heated food and nutrients retention is better than flame cooking, 5 cm of the internal heating can go deep into the food. 2, can be used as heating container material properties of nonpolar molecule material container, they do not absorb or basic rarely absorption of microwave energy, such as plastic products ( Polypropylene, polyethylene, etc. ) And glass, pottery and porcelain insulator, such as they can through the microwave, and do not absorb energy from the microwave, the material can be used as a container of microwave heating or support, or do the sealing material. Three, plastic thermal stability problem, though the plastic is not absorb energy from the microwave, but some plastic after heating food, its stability less easy to release toxic or carcinogenic substance, only cooking level using the plastic bags in the microwave heating to ensure food safety, general heating temperature not more than 120 degrees, to prevent the local temperature is too high, in the process of heating and release harmful substances, contaminated food. Conclusion: the plastic itself will be a problem in the microwave, the problem is that the heat generated by the food can make ordinary plastic bags properties change, releasing toxic substances, caused the food safety problems. So only indicate the bag, can be used in microwave heating can be heated in the microwave.