Application of microwaves in textile finishing processes

Planar microwave (MW) device for thermal treatment oftextile material was constructed and tested at Faculty of Textile Technology.The treated material, which is in a wide state, is horizontally passed throughthe slots of the applicator. The feasibility of use of developed device fortextile treatment was tested on cellulose materials impregnated with differentfinishes: durable press, water and oil-repellent and flame retardant. Obtainedeffects of microwave treatments were compared with conventional drying andcuring method and significant improvements of physical and mechanical properties were found. This might pave a way toward use of proposed technology in textilefinishing field.

Introduction

According to well known physical definition electromagnetic waves are oscillating electric and magnetic fields travelingtogether 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) [1].

The energy of microwave photons is very low (0,125kJ/mol) relative to the typical energies for chemical bonds (335-84 kJ/mol);thus MW will not directly affect the molecular structure. They cannot changethe electronic structure around atoms or among them, but they can interact withthe electronic differences between atoms.

Different materials can be divided according to theirresponse 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 toselective absorption of MW energy by polar molecules, while non-polar moleculesare inert to the MW radiation.

For a microwave electromagnetic field oscillating at2, 5 GHz, which is preferred frequency for heating applications, the chargechanges polarity nearly 5 billion times per second. Microwave radiation isspecially tuned to the natural frequency of water molecules to maximise theinteractions.

Some important applications of microwaves come fromtheir interaction with various types of material. The interaction of microwaveswith dielectric materials causes a net polarization of the substance. There areseveral different mechanisms of polarization: electronic polarization, ionic,molecular (dipole) polarization and interfacial (space-charge) polarization.The overall net polarization creates a dipole moment. Dipolerotation is an interaction, in which polar molecules or species try to alignthemselves with the rapidly changing electric field of applied radiation. Themotion of the molecule as it tries to orient to the field results in a transferof energy. The second way to transfer energy is ionic conduction that occurs ifthere are free ions or ionic species present in the substance being heated.

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.

With proper design of the waveguides and supporting equipment, a specific environment (at the particular wavelength) can be created in order to provide controlled distribution of the microwave energy, making it possible to achieve uniform exposure to material passed through a channel. The leakage of microwave energy is inherently small due to the fact that waveguide slots are oriented along the wave guideline of symmetry, and therefore they cannot act as efficient slot antennas. Furthermore, in this way the material lies in the maximum of the electric field that assures effective coupling to the flowing microwave energy. In a case that request for slots symmetry is fulfilled, only the load (textile material) which passes through the waveguides has an influence on energy loss. The amount of microwave energy absorbed by the textile in each waveguide pass depends on the material thickness and moisture content.

In a case of single pass applicator, exponential decay of electric field might cause non-uniform heat distribution. To prevent this negative tendency, the material is passed through a number of waveguide passes. Additionally, the level of applied microwave energy is increased by the use of second magnetron, that feeds the applicator at the other end. Meander type of traveling wave applicators provides uniform energy distribution across the treated material.

Before this novel device comes into commercial use, the unintentional leakage of microwave energy must be checked in order to comply with existing safety regulations of Ministry [12]. The upper limit of tolerable microwave irradiation for professional exposure is 10 W/m², or 1 W/m² in higher sensibility range. Preliminary determination of irradiation level has been performed at Department of Radio communications and Microwave Engineering at Faculty of Electrical Engineering and Computing.

Applied material and chemicals

Pre-treated cellulose fabrics, 100% cotton (desized, scoured, bleached and mercerised) of different surface mass: 105 and 250 g/m² were used in the study. The fabric samples were passed through squezze rolls on laboratory foulard, Benz, Zrih Switzerland to give wet pick up of 100% owf. Impregnation was performed with baths containing reagents for durable press finishing (baths 1-4), water and oil repellent finishing (bath 5) and flame retardant finishing (baths 6-7), shown in Table 1. After the impregnations part of the samples was dryed by conventional method in a tenter at 110 C for 2 minutes and cured in a second pasage under the producer instructions. Second part of the samples was treated by microwaves in planar microwave device with the speed of 0,5 m/min.

2.2.1. Durable Press (DP) Finishing

N-methylole based reagents, such as dimethyloldihydroxyethylene urea (DMDHEU), as well as newly developed polycarboxylic acids (PCA) represented with 1,2,3,4-butanetetracarboxylic acid (BTCA) were used in the study [13-15].

Applayed baths were containing:

DMDHEU as conventional agent with high formaldehyde (HF) content

etherified DMDHEU with low (LF) formaldehyde content

dymethylglyoxalurea (DMGU) as non-formaldehyde or free formaldehyde (FF) agent

BTCA as FF agent applied with sodium hypophosphite (SHP) catalyst.

Effects of Durable Press finishing were determined as wrinkle recovery angle (WRA) according to ISO 2313 method and tensile strength according to DIN EN ISO 1394-1. Free formaldehyde was determined by AATCC 112 method. Whiteness degree was measured at reflective spectrophotometer Datacolor - Spectraflash SF 300 at D 65/10 conditions with Data Match 300 program according to AATCC 110-2000 method.

2.2.2 Water and oil repellent finishing

Fluorocarbon polymers have been chosen to satisfy both water- and oil-repellency demands. Their main characteristic is a low surface tension (aprox. 15 mNm^-1) which causes excellent water and oil repellency effects. For good oil-repellency effects the orientation of hydrophobic atoms is of primary concern. Perfluorinated groups should be lined parallel which makes thick water and oil repellent layer [16, 17]. Because of the influence of alternating electric field it is assumed that hydrophobic fluoroalkyl groups could rotate into the polymer substrate what will decrease the repellency. For the purpose of thermal activation of hydrophobic FC groups microwave device have been modified with additional heated cylinder incorporated in the system [18]. In this study FC polymer with extender included in a formulation was applied with following concentration:

Durability of water- and oil-repellency to laundering and dry cleaning was tested after washing and cleaning cycles. Washing was performed in apparatus: Linitest, with 5 g/l of detergent and ratio 1 : 20, at 50 C, for 30 min. Dry cleaning was performed according to HRN.F.S3.027; AATCC 132-1998; ISO 105-DO1. Water repellency was tested by unstandardized Du Pont method. Tests were performed with drops of isopropanol/water mixure in ratios: from 0/100 (W as minimal degree) till 100/0 (10 as maximal degree). Oil repellency was tested according to AATCC 118-2002 (ISO 14419).

2.2.3. Flame retardant (FR) finishing

FR finishing was performed with high concentrations of organophosphorus reactant on cellulose material used for working clothes, whose surface mass was 250 g/m².

First bath (6) contains organophosphorus reactant (OFR) with conventional type of cross-linking agent based on melamine formaldehyde (MF). This type of binding agent between celullose and organophosphorus reactant was tried to be replaced with BTCA, which does not contain formaldehyde [19].

Effects of flame retardancy have been tested according to the method: ASTM D 626-68T. Durability to laundering was performed according to the Soaking test method: BS 5651:1978.

3. Results and discussion

Possibility of microwave treatment application for different finishing processes was determined in the present study.

3.1. Durable press finishing

From the results of WRA shown in tables 2 and 3 noticeable improvements can be seen in a case of treatment with microwaves. At the same time tensile strength retention has been improved, meaning that there is no usual negative interference with mechanical strength. Reasons of such an improvement have been presented earlier [20], giving an explanation with more uniform crosslinking obtained with microwaves.

According to Yang mechanical strength loss can be attributed to the choice of catalyst and its concentration rather than to the differences in the molecular structure and reagent reactivity.

Further improvement can be seen from the results of free formaldehyde release. Under the influence of microwaves formaldehyde release has been reduced by 50% compared to conventional thermal treatment what certainly gives an advantage to this method of treatment. Obtained reduction of formaldehyde content has been explained with the influence of microwaves to polar formaldehyde molecules. Because of the rapid charge changes molecules are heated resulting with expansion of the heat from the inside of the material to the surface. Liberated formaldehyde has the same flow what causes its reduction on the textile material.

Obtained results are suggesting that because of the reduction of formaldehyde release noticed on conventional N-methylole based products, which are cheaper and more reactive, but abandoned because of the formaldehyde problem, can be used. Further interesting investigation to be performed would include improvement of molecules polarity degree. In this way further formaldehyde release can be enhanced while its quantity is further reduced or completely eliminated from the treated material.

3.2. Water and -oil repellent finishing

Comparing the results of water and oil repellent finishing, shown in tables 4 and 5, improvements of water repellency are noticeable in a case of microwave treatment. With both thermal treatments effects of the oil repellency are similar.

Improvements of the repellency effects are explained with the impact of the heated cylinder, which is included in the passage of the textile material for additional improvement of FC orientation. Improvements of the repellency effects obtained with additional thermal treatment (hot-ironing) after the tenter curing have shown that the reaction was not completely finished, especially when thicker material was treated.

Effectiveness of the repellency after the dry-cleaning is very good with both applied curing methods.

3.3. Flame retardant finishing

Results of flame retardancy, showed in tables 6 and 7, obtained by microwave inducement are equally good compared to conventional curing method. Application of BTCA for binding purposes gave equaly good FR effects as conventional MF cross-linking agent which is ecologically unacceptable. Both treating baths gave good results of flame retardancy, slightly lower after the microwave treatment. Effectivenes of flame retardancy was tested after the washing, and results have revealed some decrease. Decrease of the results after the washing is more noticeable at the samples which were treated by microwaves, but they are still satisfying. Obtained durability to washing of samples treated by microwaves is confirming sufficient crosslinking of OFR molecules with celullose molecules.

Conclusions

Results of performed investigations are indicating that microwaves are suitable for drying as well as curing processes of textile finishing. From the results of textile finishing treatment with different finishing agents further conclusions can be made:

Application of microwaves in Durable Press finishing offers significant improvement in comparison to the classical curing method performed at tenter.

Microwave treatment reduces formaldehyde release.

In water and oil-repellent finishing with FC polymers MW gives certain improvements.

In flame retardant finishes with organophosphorus reagents MW are showing equally good efficiency.

Application of MW in drying and finishing process gives certain improvements of effects, but its suitability should be preliminary tested for each reagent.

Favorable effects obtained by MW and treatments cost are indicating good possibility of its implementation in praxis. After the conclusion of laboratory experiments, construction of an industrial microwave device is to be expected.

References:

1. Bartolić J.: Inenjerski priručnik: Mikrovalna elektronika, kolska knjiga Zagreb 1999, 627-717
2. Metaxas A.C., R.J. Meredith: Industrial Microwave Heating, Peter Peregrinus 1983, 111-150
3. Varma R: Solvent- free accelerated organic syntheses using microwaves, Pure Appl. Chem 73 (2001) 1, 193 - 198
4. Cablewski T. et al: Development and Application of Continuous Microwave Reactor for Organic Synthesis, .Org.Chem 59 (1994) 3408 3412
5. Englert R.D., L.P. Berriman: Curing chemically treated cellulosic fabrics, US Patent 3846845, 1974 1112
6. Bobbin: Microwaves meet wrinkle-free marketplace, October 1995 NatNews
7. anonimno: Microwave Processes for the Combined Desizing, Scouring and Bleaching of Grey Cotton Fabrics, J.Text.Institute (1996) 3, 602-607
8. Nando R., G. Patel: Microwave Oven: A tool for guide response in shade translation in reactive dyeing, Colourage 49(2002)12, 83-88
9. Reagan B.M.: Eradication of insects from wool textiles, Journal of the American Institute for Conservation 21 (1982) 2, 1-34
10. Katović D. et al: Osnove oplemenjivanja tekstila - Knjiga III, Sveučilite u Zagrebu, Zagreb, 2005, 191-192
11. Thiry M.: The Magic of Microwave, Textile Chemist and Colorist American Dyestaf Reporter 32 (19) 10, 2-4
12. Pravilnik o zatiti od elektromagnetskih polja, Ministarstvo zdravstva RH, Narodne novine, 8. Prosinca 2003.g.
13. Bischof Vukuić S. et al: Polikarboksilne kiseline u obradi protiv guvanja, Tekstil 49 (1999) 11, 549-560
14. Katović D., S. Bischof Vukuić: Application of Electromagnetic Waves in Durable Press Finishing with Polycarboxylic Acid, AATCC Review 2 (2002) 4, 39-42
15. Bischof Vukuić S. et al: Influence of Microwaves on Nonformaldehyde DP Finished Dyed Cotton Fabrics, Textile Reaearch Journal 73 (2003) 8, 733-738
16. Bischof Vukuić S. et al: Utjecaj fluorkarbonskih polimera u kombiniranim obradama pamuka s polikarboksilnim kiselinama, Tekstil 53 (2004) 3, 103-109
17. Bischof Vukuić S., D. Katović: Textile finishing treatments influenced with microwaves, The Textile Institute 83^rd World Conference, Shangai, May 23-27, 2004, 1165-1169
18. Bischof Vukuić S. et al: Effect of microwave treatment on fluorocarbon finishing Colourage Annual 51 (2004) 1000 -1004
19. Yang C.,W.Wu: Combination of a hydroxy-functional organophosphorus oligomer and a multifunctional carboxylic acid as a flame retardant finishing system for cotton, Fire and materials 27 (2003) 223-237
20. Kang I.S. et al: Mechanical Strength of Durable Press Finished Cotton Fabrics, textile Research Journal 68 (1998) 11, 865-870

List of Tables

Tab.1 Applied agents and treatment conditions

Tab. 2 Effects of Durable Press finishing on cotton material (105 g/m²) treated conventionally

Tab. 3 Effects of Durable Press finishing on cotton material (105 g/m²) treated by microwaves

Tab. 4 Effects of water and oil-repellent finishing on both cotton materials treated conventionally

* Additional thermal treatment

Tab. 5 Effects of water and oil-repellent finishing on both cotton materials treated by microwaves

* Additional thermal treatment

Tab. 6 Effects of flame retardant finishing on cotton material (250 g/m²) treated conventionally

Tab. 7 Effects of flame retardant finishing on cotton material (250 g/m²) treated by microwaves

About the Authors:

Prof. Drago Katovic, Sandra Bischof Vukusic and Sandra Flincec Grgac are associated with the Faculty of Textile Technology, University of Zagreb, Department of Textile Chemistry and Ecology.

This paper is original scientific paper, UDK.

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