Experimental
The microwave heater used was the one presented in [1]. 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.
Discussion
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 [4]. 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
Results
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 [5].
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].
Conclusion
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.
References
[1]Haghi, A.K., 2003, Heat and Mass Transport Through Moist Porous Materials, 14th Int. Symp. On Transport
Phenomena, 209-214. .
[2] Robinson, T.G., McMullan, G.R., Marchant. P., 2001, Remediation of dyes in textile effluent: a critical review
on current treatment technologies with a proposed alternative, Journal of Bioresource Technology, 77, 247-255.
[3] Lin, S.H., and Liu, W.Y., 1994, Continuous Treatment of Textile Water by Ozonation and Coagulation,
Journal of Environmental Engineering, , 120(2), 78-89.
[4] Bell, J., Plumb, C., Buckley, A., and Struckey, D., 2000, Treatment and declorization of Dyes in an Anaerobic Baffled Reactor, Journal of Environmental Engineering, (126)11,42-51.
[5] Wu, J., Etiman, A. and Law, S.1998, Evaluation of Membrane Filtration and Ozonation Process for
Treatment of Reactive Dye Wastewater, Journal of Environmental Engineering, (124)3,24-35.
[6] Haghi, A.K, 2003, The Diffusion of Heat and Moisture Through Textiles, International Journal of Applied
Mechanics and Engineering, , Vol. 8, No. 2, 233-243.
[7] Haghi, A.K., 2003, Drying Process- A Theoretical Approach , J. of Thermal Analysis and Calorimetry, To
be Published in volume 74.
[8] Haghi, A.K., 2003, The Diffusion of Heat and Moisture Through Textiles, International Journal of Applied Mechanics and Engineering, Vol. 8, No. 2, 233-243.
To read more articles on Textile, Industry, Technical Textile, Dyes & Chemicals, Machinery, Fashion, Apparel, Technology, Retail, Leather, Footwear & Jewellery, Software and General please visit https://articles.fibre2fashion.com
To promote your company, product and services via promotional aticle, follow
this link: https://www.fibre2fashion.com/services/article-writing-service/content-promotion-services.asp
Comments