Planar microwave (MW) device for thermal treatment of textile material was constructed and tested at Faculty of Textile Technology. The treated material, which is in a wide state, is horizontally passed through the slots of the applicator. The feasibility of use of developed device for textile treatment was tested on cellulose materials impregnated with different finishes: durable press, water and oil-repellent and flame retardant. Obtained effects of microwave treatments were compared with conventional drying and curing method and significant improvements of physical and mechanical properties were found. This might pave a way toward use of proposed technology in textile finishing field.
According to well known physical definition electromagnetic waves are oscillating electric and magnetic fields traveling together through space. In the electromagnetic radiation spectrum, shown at figure 1, microwaves (300 MHz 300 GHz) lie between radiowave (Rf) and infrared (IR) frequencies, with relatively large wavelength (1m-1mm) .
The energy of microwave photons is very low (0,125 kJ/mol) relative to the typical energies for chemical bonds (335-84 kJ/mol); thus MW will not directly affect the molecular structure. They cannot change the electronic structure around atoms or among them, but they can interact with the electronic differences between atoms.
Different materials can be divided according to their response on microwave radiation:
The materials that reflect MW radiation (stayed cold)
The materials that is transparent to MW radiation (non-heated)
The materials that absorb MW energy (being heated).
However, chemical reactions can be accelerated due to selective absorption of MW energy by polar molecules, while non-polar molecules are inert to the MW radiation.
For a microwave electromagnetic field oscillating at 2, 5 GHz, which is preferred frequency for heating applications, the charge changes polarity nearly 5 billion times per second. Microwave radiation is specially tuned to the natural frequency of water molecules to maximise the interactions.
Some important applications of microwaves come from their interaction with various types of material. The interaction of microwaves with dielectric materials causes a net polarization of the substance. There are several different mechanisms of polarization: electronic polarization, ionic, molecular (dipole) polarization and interfacial (space-charge) polarization. The overall net polarization creates a dipole moment. Dipole rotation is an interaction, in which polar molecules or species try to align themselves with the rapidly changing electric field of applied radiation. The motion of the molecule as it tries to orient to the field results in a transfer of energy. The second way to transfer energy is ionic conduction that occurs if there are free ions or ionic species present in the substance being heated.