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Quality of modified flax fibers.

Abstract: When polycarboxil acid affects cellulose, a chemical modification of molecular and also of super molecular fiber structure occurs, making natural fibers modified in such a way that they become interesting for use in new textiles where they can substitute man-made materials on an equal basis. In this paper a chemical modification of flax fabric has been performed using citric acid, and a physical one, using ultrasound.

Key words: flax fabric, quality, citric acid, modification

1. INTRODUCTION

Since lignocelluloses and especially flax fibers are being increasingly used for clothing and technical textiles, the need for the development of objective methods for the valorization of fiber quality as well as the improvement of the available and finding new modification procedures with the aim of promoting their properties and eliminating their deficiencies has been felt. Good mechanical properties, thermo stability, biodegradability and origin from sustainable resources ensure a breakthrough of lignocellulose fibers into the field of so-called composites. Thus, they are becoming increasingly interesting for the use in procedures where they can substitute man-made materials on an equal level. Flax and other lignocellulose fibers have similar mechanical properties as glass and some carbon fibers, but due to micro morphological characteristics and lower density they have a rough surface and a specific layered structure, they are air-permeable and absorptive, they do not cause allergic reactions, and they are price worthier than man-made fibers. They afford advantage, because they do not cause additional environmental pollution. Their manufacture requires less energy consumption than man-made fibers. Problems arising during processing and using lignocellulose fibers, such as markedly variable fiber quality (fineness, length), poor bending strength, moisture absorption and as a consequence undesirable rotting, and in composites an insufficient adhesiveness with the matrix can be solved by various physical-chemical interventions at the level of morphology and fiber structure (Surina et al., 2006). Thus, less water absorption in cellulose fibers is obtained by hemicelluloses extraction, cross-linking of very absorptive degraded products of hemicelluloses and lignin, and by increasing cellulose crystallinity. Fiber fineness uniformity is achieved by cottonization, and aesthetics and hand are improved. One of the possibilities is chemical modification of native cellulose with mono- and polycarboxil acids. By using appropriate catalysts, polycarboxil acids, including citric acid (CA), are cross-linked with cellulose by an esterification mechanism. Acetylation makes fibers become less prone to moisture and water absorption, they swell less and are more resistant to microorganisms, their mechanical properties and abrasion resistance change (Bischof Vukusiae, et al., 1999; Sefc, 2006; Weilin, et al., 2001). In treating the cotton fabric with citric acid positive results were obtained in relation to crease resistance. At the same time, mechanical properties (breaking strength and abrasion resistance) were reduced, so that for the purpose of this work, after treating the flax fabric with citric acid, an additional ultrasound treatment was performed in order to improve the modification method with polycarboxyl acid.

2. EXPERIMENTAL

2.1 Materials and measuring procedure

Investigations included a flax fabric with a mass per unit area of 270 g/m2. After treating with citric acid with a concentration of 7% and addition of the catalyst sodium phosphate dibase ([Na.sub.2]HP[O.sub.4] x 12[H.sub.2]O) with a concentration of 6.5% (sample CA), the same sample was exposed to ultrasound (sample CA-US). To verify the efficiency of the mentioned treatment, the treated samples were compared to a non- treated sample (sample N) as well as to the non-treated sample, which had been additionally exposed to ultrasound (sample N-US).

The fabric was impregnated using the Benz laboratory padder with a squeezing effect of 100%. After impregnation the fabric was dried at 110 [degrees]C for 2 minutes and thermally treated in the Benz stenter for the period of 90 seconds at a temperature of 180 [degrees]C. After the modification with citric acid the samples were exposed to ultrasound. Treatment duration for all samples was 15 minutes at room temperature in water medium.

2.2 Test methods

Tests of relevant properties in order to monitor the influence of citric acid and ultrasound on the properties of flax fabric were made according to standardized methods as follows: thickness (HRN F. S2. 021), thread density (EN 1049-2:1973), mass per unit area (EN 12127:1997), air permeability (EN ISO 9237), tensile strength and deformation (UNI EN ISO 2062), wrinkle recovery angle (ISO 9867:1991) and abrasion resistance (ISO12947-3). Prior to all tests, samples were conditioned at 20[degrees]C and 65% RH (standard conditions) for a minimum of 48 hours.

3. RESULTS

Table 1 shows test results for construction parameters and air permeability of the non-treated and treated samples, as well as of the samples additionally exposed to ultrasound. Table 2 shows test results for wrinkle recovery angle of the non-treated and treated samples as well as of the samples additionally exposed to ultrasound. Results for abrasion resistance (Table 3) are shown in terms of mass loss, %, after 10,000 cycles with a preload of 9 kPa. Test results for tensile strength of the non-treated and treated samples are shown in Fig. 1 and elongations at break are shown in Fig. 2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

4. DISCUSSION AND CONCLUSION

The analysis of the construction parameters (Table 1) indicates that, as a result of cross linking the cellulose molecules with citric acid, slight changes in fabric thickness and thread density are caused. The mass per unit area of the modified fabric was increased indicating sample shrinkage. A certain discrepancy of measurement results between construction parameters (thickness, thread density, mass per unit area) is the consequence of quality variability (fineness), which is a characteristic of flax fibers. A conclusion may be drawn that the modification with citric acid under the described conditions does not significantly influence construction parameters. An additional exposure of the non-treated sample and the sample treated with citric acid to ultrasound causes certain changes of construction parameters, and they are expressed as a slight increase of thread thickness and density and a reduction of mass per unit area (N-US and CA-US).

Air permeability of the non-treated flax fabric amounts to 639.1 mm/s, but after exposure of the sample to ultrasound, lower air permeability was measured (Table 2). After treating the sample with citric acid, a high reduction of air permeability was achieved, in contrast to the non-treated sample, and after an additional exposure to ultrasound, air permeability increased by 15% in comparison to the sample treated with citric acid. This brings to the conclusion that citric acid caused the whole textile structure to be bonded at the molecular and morphological level. It may be assumed that ultrasonic waves reduced the achieved effect of cross--linking, the result of which is the improvement of air permeability.

While testing abrasion resistance, a positive influence of the treatment with citric acid was noticed as well as of the additional exposure to ultrasound. The non-treated sample exposed to ultrasound (N-US) is more resistant to abrasion and mass loss is lower by 15%, respectively. The sample treated with citric acid (CA) has even a higher abrasion resistance (mass loss is 24%). The highest abrasion resistance in contrast to the other samples was measured in the CA-US sample.

The results of wrinkle recovery angle measurements did not confirm the expected increase of the resistance to bending of the chemically modified flax fabric. It is noted that ultrasound yields an increase of jump angle and quality number in the warp direction by 50% and in the weft direction by approximately 20% in comparison with the untreated (N) and chemically modified sample (CA).

After exposure to ultrasound, the tensile strength of the non-treated sample increased by 40%. Likewise, but to a lesser extent, the tensile strength of the chemically modified sample increased. By successive implementation of chemical and physical modification, an even higher resistance to force application (Fig. 1) is noticed. The analysis of the numerical values of deformations (Fig. 2), obtained by measuring the tensile force, indicates that the chemical and physical modification increases the sample deformation, whereby the effects of ultrasound (N-US, CA) took a higher impact. The after-effect of ultrasound on the chemically modified sample (physical-chemical modification) reduced the deformation (CA-US).

Physical-chemical intervention with citric acid took no effect on the construction parameters but, on the other hand, an improvement of mechanical properties (tensile strength and abrasion resistance) was found. For the continuation of this research it is planned to carry out a chemical modification under other conditions and to explore the effect of the modification on sorption properties and microbiological resistance of flax fibers.

5. REFERENCES

Surina, R.; Andrassy, M. & Pezelj E. (2006). Technical and Cottonised Flax Fibers - Comparison of Properties--37th International Symposium on Novelties in Textiles, Book of Abstracts Ljubljana

Bischof Vukusiae, S.; Katoviae, D. & Soljaeiae, I. (1999). Polikarbosilne kiseline u obradi protiv guzvanja, Tekstil 48 (11), 549-560, ISSN 0492-5882

Sefc, B. (2006). Utjecaj obrade drva limunskom kiselinom na njegova svojstva, doktorska disertacija, Sumarski fakultet, Sveueilista u Zagrebu

Weilin Xu, Weigang Cui, Wenbin Li & Weiqi Guo (2001). Two-step durable press treatment of cotton fabric, Coloration Technology, 117 (6), 352-355
Table 1. Results of determining construction parameter and air
permeability of non-treated and treated samples

tested properties N N-US CA CA-US

thickness [mm] 0,60 0,61 0,53 0,60
warp thread density warp 15,0 15,0 14,0 15,5
on 1 cm weft 14,0 14,0 13,0 15,0
mass per unit area [g/[m.sup.2]] 273,8 267,5 285,2 270,5
air permeability [mm/s] 639,1 597,2 471,1 546,9

Table 2. Results of determining wrinkle recovery angle of nontreated
and treated samples

samples [alpha] [[??]] [alpha]0 [[??]] K [%]

 5' 60'

N warp 39 49,6 21,4 3,3
 weft 44 61 19,4 3,7
N-US warp 39 58 14,4 2,6
 weft 50 58 37,5 6,2
CA warp 40 58 15,8 2,8
 weft 43 56 22,2 3,8
CA-US warp 57 70 34,1 7,4
 weft 44 55 25,2 4,3

Table 3. Results of determining abrasion resistance of non-treated
and treated samples

 no ultrasound with ultrasound
 treatment treatment

 loss of test loss of test
 mass interval mass interval
samples [%] (rubs) [%] (rubs)

N 9,1 10000 7,7 10000
CA 6,9 10000 4,6 10000
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Author:Surina, Ruzica; Andrassy, Maja
Publication:Annals of DAAAM & Proceedings
Geographic Code:4EUAU
Date:Jan 1, 2007
Words:1724
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