One must consider that the material within the pillow is capable of self-heating under a specific set of conditions.

Classical self-heating theory pertains to the transfer of heat and oxidizer from the ambient air to the inside of a porous mass of material. The transfer of heat from the outside air to the interior of the pile of material (conduction) provides energy for the chemical reaction that is the heat source in the self-heating process.

The porous material provides thermal insulation that can, in some cases, allow a net increase in energy (rate of energy generated is greater than rate of energy loss) and a resulting increase in temperature within the pile of material. Since the rate of chemical reaction within the interior of the pile of material is temperature dependent and the pile is thermal insulated, the highest temperatures are nominally associated with the "center" of the pile of material.

This sounds like a classic case of self-heating, but with a difference in how the energy is delivered to the center of the pile of material. The heating of material in a microwave oven tends to produce higher temperatures within the interior of the material without the need to transfer its heat from the outside of the pile through the interior of the pile through conductive heat transfer. Thus, the microwave oven delivers sufficient energy to the interior of the pile to allow the self-heating process to occur.

The fabric cover provides additional thermal insulation at the surface of the material that promotes self-heating of the material. The "delay" in the noticeable occurrence of the incident is also consistent with self-heating since the driving force is heat transfer, which is time dependent and does not an instantaneous ignition source.

I hope this is helpful to you in your investigation.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com