Understanding microwaves and the Reverse Polymerization (RP) process is valuable for appreciating the advantages of this technology over other possible solutions. Microwaves are electromagnetic waves in the frequency band from 300 MHz to 300 GHz. Industrial microwave processing is usually accomplished at frequencies of 915 MHz or 2,450 MHz.
When an electric field, such as can be generated by microwave energy, interacts with a material, a number of responses can take place:
- 1. In a material that is a conductor, electrons move freely in the material in response to an electric field and an electric current results. Unless the material is a superconductor, the flow of electrons will heat the material through resistive heating. Resistive heating is the process that is used in a typical electric heating element such as a stovetop.
- 2.In an insulator, electrons do not flow freely, but re-orientation or distortions of induced, or permanent dipoles, can give rise to heating.
Microwaves penetrate materials and release their energy in the form of heat as the polar molecules (ones with positively and negatively charged ends – such as water) vibrate at high frequency to align themselves with the frequency of the microwave field. The microwaves interact directly with the object being heated. The interaction is related to the chemical properties of the object and it is possible to apply heat in ways that can not be achieved by conventional means (convection heating, conductive heating or radiant heating). For example, it is possible to heat up one object whilst another, close by, remains relatively cool. An example is a typical home microwave oven that will heat and cook the food, generally containing water, while the glass plate stays relatively cool. The water in the food links with the microwave energy while the plate is invisible to the microwaves. Any heating of the plate is caused by radiant heat transfer from the hot food. Microwave heating also heats the food relatively uniformly throughout while radiant or convection heating, raises the temperature from the outside inward. See the figure below for a comparison of the heating processes.
How Microwaves Work
Microwaves are similar to sound since they travel in waves. Microwaves are different then sound because they can travel through space, whether air is present or not, and rather than creating slight pressure differences, microwaves create electromagnetic waves. These electromagnetic waves are common in our everyday life:
- Light is an electromagnetic wave
- AM and FM radio stations transmit their signals through electromagnetic waves
- Cell phones and wireless home networks use electromagnetic waves to send and receive data
Microwaves are largely reflected from metallic conductors, but interact well with dipoles (asymmetrically charged molecules), such as water. Microwaves are an efficient method of heating non-conducting materials know as dielectrics, which are not heated as efficiently by conventional convective methods. Microwaves generate rapidly changing electric fields and dipoles rapidly change their orientations in response to the changing fields. If the field change is occurring near the natural frequency at which reorientation occurs, then a maximum utilization of energy is realized and optimum heating occurs. The material is then said to be ‘well coupled’ with the microwaves.
The material properties of greatest importance for insulators, or dielectrics, are the permittivity and loss factor (measures of the materials ability to absorb and store energy) and especially a factor termed tan delta-d (the materials ability to convert stored energy into heat). Hence, for optimum coupling a balanced combination of moderate permittivity and high loss/tan delta-d is required.
At the microwave frequencies of 915 MHz or 2,450 MHz, the magnetron is the economic product of choice for the generation of the required power and frequency. The magnetron is a diode-type electron tube, which is used to convert high voltage DC power under the influence of strong permanent magnets into the microwave frequencies and power of choice. At 2,450 MHz, which is the frequency used at EWS, magnetron powers from 0.2-10 kW can be obtained. The EWS RP process uses arrays of magnetrons with variable power output to a maximum of 3 kW each, depending on the application.