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.
The microwave heater used was the one presented in . Batch dyeing was carried out using a laboratory padder. Some control dyeing for comparative purposes were done in an Ahiba dyeing machine. All dyes and chemicals used were commercial products. A 100% cotton woven fabric previously prepared for dyeing was used. Colour easurement and analysis of calorimetric data were carried out on spectrophotometer.
Control dyeings for comparative purposes were carried out using the procedure recommended by the vendors of the dyes. A cold-batch procedure was followed for the control dyeing with Vinylsulphone and VS/MCT (Vinylsulphone/Monochlorotriazine) dyes while an exhaust procedure at 100 C was adopted for the control dyeing with nicotinic acid dyes. Unfixed dye was washed off immediately after the dye fixation stage was completed. Wash-off consisted of rinsing the fabric twice in 100 times its weight of water for 5 minutes at 90C, soaping in an anionic detergent solution and rinsing in water under fixation of reactive dyes with the same conditions. Virtually no dye was removed by the final rinse.
The dye was applied by padding as in a conventional pad-batch process, and the fabric batched on a roll. The fabric roll was wrapped with a single thickness of corrugated carboard, and the exposed ends of the roll insulated with polyurethane foam. The fabric is then placed in the microwave while being heated to a temperature of approximately 90 C. The fabric then washed as in conventional dyeing process.
Pertinent electromagnetic parameters governing the microwave heating
The loss tangent can be derived from materials complex permittivity. The real component of the permittivity is called the dielectric constant whilst the imaginary component is referred to as the loss factor. The ratio of the loss factor to the dielectric constant is the loss tangent. The complex dielectric constant is given by:
ε = ε'− jε" _____________________________ (1)
Where ε is the complex permittivity, ε ' is the real part of dielectric constant; ε " is the loss factor, and ε'ε"= tanδ is the loss tangent.
Knowledge of a materials dielectric properties enables the prediction of its ability to absorb energy when exposed to microwave radiation. The average power absorbed by a given volume of material when heated dielectrically is given by the equation:
P av =ϖ ε 0 ε eff E rms 2V __________________ (2)
Where Pav is the average power absorbed (W); ϖ is the angular frequency of the generator (rad/s); ε 0 is the permittivity of free space; ε eff " is the effective loss factor; E is the electric field strength (V/m); and V is the volume (m3).
The effective loss factor ε eff includes the effects of conductivity in addition to the losses due to polarization. It provides an adequate measure of total loss, since the mechanisms contributing to losses are usually difficult to isolate in most circumstances.
Another important factor in dielectric heating is the depth of penetration of the radiation because an even field distribution in a material is essential for the uniform heating. The properties that most strongly influence penetration depth are the dielectric properties of the material. These may vary with the free space wavelength and frequency of the propagating wave. For low loss dielectrics such as plastics the penetration depth is given approximately by:
DP = λ0 ε'
2 πε eff ________________________ (3)
where DP is the penetration depth; λ 0 is the free space wavelength; ε ' is the dielectric constant; and "ε eff is the effective loss factor.
The penetration depth increases linearly with respect to the wavelength, and also increases as the loss factor decreases. Despite this, however, penetration is not influenced significantly when increasing frequencies are used because the loss factor also drops away maintaining a reasonable balance in the above equation. As the material is heated, its moisture content decreases leading to a decrease in the loss factor. It can be seen from equation (3) that the decrease in loss factor causes in the penetration depth of radiation.
In heating with microwave irradiation under fixed conditions, water is evaporated from the exposed yarn surfaces and replaced by water migrating from within the fabric structure. During this process, a steady state is established and the fabric temperature and evaporation rate remain almost constant [3[. At the end of this socalled constant rate heating period, the fabric temperature begins to increase and the rate of evaporation decreases, because below certain critical water content, migration of liquid water, and later water vapor, to the exposed surfaces becomes more difficult and eventually ceases.
It has been recognized that microwave could perform a useful function in textile heating in the leveling out of moisture profiles across a wet sample. This is not surprising because water is more reactive than any other material to dielectric heating so that water removal is accelerated . Microwave heating may make more pronounced the non-uniform temperature distribution with certain geometries. This leads to giving a temperature gradient inside the leather sample with the opposite directions to that in conventional drying process. The temperature inside the fabric slab becomes higher than it is on the surface so the diffusion and thermo-diffusion gradient lead in the same direction and dewatering rate increases.
Fig.1. Apparent colour strength of reactive dyes applied to cotton
The levels of fixation of dyes in batch processes can be dramatically accelerated by heating the fabric during the batching stage (Figure 1). Microwave heating of the fabric can lower the time required for fixation of reactive dyes.
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 .One of the main features which distinguishes microwave heating from convective heating process is that because liquids absorb the bulk of the electromagnetic energy at microwave frequencies, the energy is transmitted directly to the wet material. The process does not rely on conduction of heat from the surface of the product and thus increased heat transfer occurs, speeding up the heating process. This has the advantage of eliminating case hardening of the products which is usually associated with convective hot air heating operations.
Another feature is the large increase in the dielectric loss factor with moisture content. This can be used with great effect to produce a moisture leveling phenomenon during the drying process since the electromagnetic energy will selectively or preferentially dry the wettest regions .
In this study the microwave irradiation on samples at different heating power level was achieved. It is seen that the incident power strongly influenced the drying kinetics of samples, reducing the heating time by raising the microwave heating power. All samples show the same behavior with dependence on applied microwave power.
The influence of the power level is clearly defined. It seems that the high intensity of heating due to overheating the surface layer is making it difficult for the diffusion process of mass transfer through the material. This effect is marked for all samples investigated.
The maximum heating rate is established immediately at the beginning of the process because the initial moisture content was below the critical sample moisture content and there was sufficient microwave power in the cavity.
The microwave heating of samples showed the higher heating rates and consequently faster heat-up times, higher temperature and pressure gradients within the material. At this stage a pumping effect within the material becomes apparent. This pumping effect is primarily caused by an internal pressure buildup which forces the free liquid in the pores of the samples. Although this pumping action does increase heating rate, the increased temperature and pressure that occur can cause damage to the material.
This study opens the possibility for further investigations on microwave heating of textiles [6-8].
In this experimental investigation, reactive dyes with different types of functional groups did not behave the same in the hot pad-batch dyeing method. Vinylsulphone types produced lower colour yields when applied using the hot method compared with those obtained with the conventional cold method.
Dyes with a nicotinic acid leaving group gave colour yields equivalent to those in the recommended batchwise method when applied by the hot pad-batch method. These dyes cannot be applied using cold-batch methods.
Vinylsulphone/ monochlorotriazine bifunctional reactive dyes gave excellent results using hot pad-batch method. Colour yield and colour fastness to washing were superior to those obtained using the recommended cold pad-batch process.
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