The main difference between conventional heating with hot air and microwave heating is the heating mechanism. While conventional techniques heat a surface, the microwaves heat the whole volume of the treated object. During the conventional heating, the heat is generated outside the treated product and conveyed by conduction or convection. Hence, the surface is heated at first and afterwards the heat flows toward the inside, which always remains colder than the surface. The required internal temperature can be reached only by sufficient increase of the surface temperature of the material above the temperature needed for particular treatment.

On the contrary, in MW treatment, the heat is generated in a distributed manner inside of the material, allowing more uniform and faster heating. According to the literature [2], the energy consumption is 60-70 % lower in a case of microwave treatment.

Term "microwaves" was used for the first time in 1932^nd, and its first usage was during the Second World War in radiocommunication and radar technology. The activity of electromagnetic field of high frequency was discovered accidentally during a radar-related research project, while testing a new vacuum tube, called a magnetron. Until now, MW have been used for food preparation, chemical sludge, medical waste, organic synthesis, analytics and curing of hi-tech polymers [3, 4]. Today they are widely accepted and spread to mobile phones, television, wireless computer networks and some special applications such as rocket engines.

Electromagnetic waves have been used in the textile finishing for the purpose of drying of thick materials, performed at radio frequency (RF) dryers which are operating at different frequencies. This kind of dryers are operating at frequencies of 27,12 MHz with power from 10 till 100 kW.

First idea of MW application for textile finishing processes originated in 1970-es when cellulose fabrics were treated with Durable Press (DP) finishing agents and cured in MW oven [5]. Although these first results were promising, the idea was abandoned till 1955, when Miller [6] patented his Pre-set process without awareness of the earlier patent. Both cases involved garment microwave treatment, but they were abandoned because of efforts to control the process failed. Until now, MW irradiation for textile finishing has been used for the combined desizing, scouring and bleaching processes [7], dyeing [8] and drying processes, as well as for eradication of insects from wool textiles [9]. Additional usage was for continuous measuring of low humidity [10]. All this experiments were performed in a resonant cavity. Completely different system was used in microwave device constructed by american firm Industrial Microwave System (IMS). Treated material is passed through the waveguides in a rope state [11].

Experimental

Main idea of microwave device construction was to treat textile material on continous flow basis. It has been achieved by passing the textile material through a slots of waveguide-based applicator. Experiments were performed at textile material treated with different finishing processes. Cellulose material was impregnated with baths 1-7, and passed through a waveguides.

 

Microwave device

Laboratory microwave device, shown in figure 2, was constructed at the Department for Textile Chemistry and Material Testing of Faculty of Textile Technology, University of Zagreb. This novel system offers passage of textile material in a wide state through a waveguides. The system consists of 6 centrally sloted rectangular waveguides (dimensions 4 x 8 cm) and 2 magnetrons fed by 500 W. Waveguide is terminated with water-based dummy load that prevents leakage of residual microwave energy.