An experimental study was undertaken to investigate the effect of microwave heating on dyeing polyester. The results obtained show a high increase in dye uptake and dyeing rate acceleration. Performance of dye leveling and colour homogeneity was achieved, which was found to be better than obtained by conductive heating.
While cost presents a major barrier to wider use of microwave in textile industry, an equally important barrier is the lack of understanding of how microwaves interact with materials during heating process. Microwave energy has several possible benefits in textile processing . Substitution of conventional heating methods by microwave irradiation may result in faster and more uniform heating, more compact processing machinery requiring less space, and less material in process at a particular time.
Microwave heating in dyeing processes has been studied for many years, but only a limited amount of information has been published. The objective of the present study is an attempt to investigate more over the research paper presented in  on the effect of microwave irradiation. The only difference is that the microwave heating was used during the batching stage to accelerate the fixation of reactive dyes on cotton in pad-batch dyeing.
In textile processing it is necessary to apply heat as in dye fixation, heat setting. Heat can be transferred to material by radiation, conduction and convection. These three ways of transferring can be used either separately or in combination. The saving of time and energy is of immediate interest to textile industry. The introduction of new techniques which will allow less energy to be used is a highly important area of activity to consider. A relatively short section of properly designed microwave heating can increase production speeds.
Moisture in a simple porous body is held within the voids of the solid matrix. On heating process, capillary-driven flows empty the larger pores, often maintaining the exposed surface sufficiently wet for the drying to be controlled entirely by the moisture/vapor transport through the surrounding air.
The heating process of textiles is more complex, as moisture migration in fibrous masses and web can take place in a number of ways, e.g. movement due to capillarity and gravity within the inter-fibre spaces, liquid diffusion along the fibers themselves due to moisture and temperature gradients, and vapor diffusion throughout the voids in the mass. Adsorbed moisture may be stripped through effusion when the mean free path of the molecules is of similar dimensions to the diffusional space. This kind of moisture removal, however, is unlikely under most commercial drying conditions [2-5]. Any surface diffusion of sorbed moisture may not significantly influence the overall transport of moisture, as the migration material may simply re-circulate around a single air-filled pocket. Nevertheless, transport of sorbed moisture through cellulosic fibers does appear to be possible.
It is normally assumed that large fiber masses and webs are microscopically homogeneous, so that it is possible to apply conservation and constitutive equations over sufficiently small control volumes to obtain smooth profiles of temperature and moisture content. Should no detailed information on these profiles be required, then it is possible to fit a simple diffusion equation to the drying process, often with a concentration-dependent diffusion coefficient.
Migration of dye can take place during the heating of fabric padded with a dispersion of dye in water. Such migration can lead to shading problems in the finished fabric. Initially the pore network in the fabric is composed of capillaries of larger size than the dye particles, and the dye dispersion can freely move with the liquid moisture. As heating proceeds the remaining capillary sizes become smaller and liquid discontinuities more numerous, thus progressively stranding the dye particles. As a result there exists for every fabric a critical regain below which there can be no migration. To mitigate the problem liquor pick-ups are minimized and thickeners added during the dyeing stage.
Method Of Study And Assumptions
In microwave heating, the most important variable in determining the power absorption is the loss factor, which is fixed by the electrical properties of the material. The effective loss factor of a wet material is derived from the solid matrix, the bound water and the free water. The latter is dominant at the higher moisture content and is derived from the ionic conductivity of dissolved salts and the loss due to the rotation of dipolar molecules in the applied electrical field. In the absence of dissolved salts, the loss factor is a maximum in the microwave region at 17 Ghz. . Compared with conventional drying systems, which involve circulating hot air through or around the package, microwave heating produces an even moistness in the dried packages, causing less variation in the winding tension. Some overheating of material normally occurs with conventional convective heating.