Peracetic acid is more effective as a bleaching agent than hydrogen peroxide and an acceptable degree of whiteness (CIE whiteness index 80) can be obtained with minimum loss of tensile strength, reveals Dr A Farhan Khan.

The art of bleaching has been practised since beginning of civilization [1,2]. Bleaching is the process of removing coloured impurities from the griege fabric as efficiently as possible, with minimum or no damage to the fibre and leaving in a perfect white state [3,4].

Nowadays, consumers increasingly demand more environmentally friendly products. This also affects the textile industry, and thus, aspects such as control of water, energy and chemicals consumption should be taken into account in wet textile processes. Hydrogen peroxide (H2O2) is the most widely used bleaching agent for textiles and came into use around 1878[1]. Hydrogen peroxide is suitable for most fibres and it can be used in a wide range of machines under different conditions. Reaction products are non-toxic and non-dangerous but hydrogen peroxide is a highly corrosive compound and degrades to oxygen and water. Hydrogen peroxide is however, damaging to fibre, because it is applicable in strongly alkaline medium and it requires a high temperature to give the most effective bleaching [5,6].

Paracetic Acid (PAA) as a bleaching agent has many advantages compared to hydrogen peroxide. It does not produce any toxic byproducts in the bleach reaction, it is less corrosive, it is biologically totally degradable and it causes no AOX (absorbable halogenated organic compounds) load in the waste water [7].

Paracetic acid can be prepared in situ in solution from hydrogen peroxide and acetic anhydride.
H2O2 + (CH3CO)2O ---> CH3COOOH + CH3COOH

Commercial peracetic acid, which is available, for example, in 5% and 15% solutions, is a colourless liquid with a pungent smell and both solutions are water soluble[8].

Experimental
Materials
Fabric
The characteristic parameters of the 100% pure scoured cotton fabric used for all the experiments, purchased from the market are presented in Table 1.

Water:
The water used during all bleaching and washing operations had the following qualities.
The Total Hardness was measured in terms of calcium carbonate. The pH, Total Hardness and Total Dissolve Solids (TDS) of water suitable for all textile processing are 6.5-7.5, 0-50 ppm and 65-150 ppm respectively [9].

Equipment/Method
Bleaching Machine
Bleaching runs were carried out in an SDL ‘ECO’ Infra Red Lab Bleaching/Dyeing machine with automatic temperature programming and agitation.

Digital pH Meter:
A digital pH/Temperature meter was used with a combination of glass electrode.

Whiteness Measurement:
The CIE Whiteness Index value (CIE WI) was determined for the bleached fabric using AATCC Test method (110–1995) [10]. The whiteness was measured using a DatacolourSpectra flash SF 600X with the following setting; illuminants D-65, large area view, specular included and CIE 1964 supplemental standard observer (100 observer). Each sample was folded twice to give an opaque sample with four piles and the whiteness was measured four times at different fabric surface. The average value of (CIE WI) was recorded.

Absorbency:
Absorbency was determined as per AATCC Test Method (79-1986) [11]. Absorbency is one of the several factors that determine the suitability of a fabric for a particular use wet ability or absorbency of textiles or fabric can be determined by the this test method.

Fluidity:
Ostwald Cannon-Fenske (Cuen) (Cupriethylene Diamine Hydroxide) viscosimeter was used to determine the chemical degradation of cotton by measurement of their fluidity as per AATCC Test Method (82-1989) [11].

Tensile Strength:
The tensile strength was measured by Universal Strength Tester (Titan) according to EN (ISO. 13934-1: 1999) [12].

Chemicals:
Hydrogen Peroxide:
Hydrogen Peroxide (50% wt/wt) supplied by MERCK (Germany).
Wetting Agent:
Sandozin Niti in liq (non ionic) wetting agent supplied by Clariant (Pakistan).
Sodium Hydroxide:
Sodium Hydroxide (NaOH) pellets supplied by MERCK (Germany).
Peracetic Acid:
Peracetic acid supplied by Tianjin Xinyuan Chemical, CO., Ltd (China).
Stabilizer EDTA:
Stabilizer EDTA supplied by MERCK (Germany).

For comparing the hydrogen peroxide and peraectic acid bleaching effects, the recipes used are shown in Table 3.

The hydrogen peroxide and peracetic acid bleached samples were then hot-washed at 95°C for 15 minutes followed by cold wash and air dried.

Results and Discussion
The purpose of this comparative study was to explore the possibility of bleaching cotton fabric by peracetic acid and to achieve an acceptable degree of whiteness (CIE whiteness index 80) with minimum loss of tensile strength and maximum absorbency. The results of CIE whiteness index, tensile strength, absorbency and fluidity are shown in Table 4.

The colouring matter present in cotton is characterized by the presence of conjugated double bonds and these double bonds are attacked by the oxidizing species during bleaching [13,14]. Bleaching was carried out with PAA (3g/l) on scoured cotton fabric at pH-7 for 40 minutes. The PAA bleached sample was compared with sample bleached by hydrogen peroxide. It was observed that PAA bleaching increased the CIE whiteness index from 28.9 (non bleached cotton fabric) to 83.6, this whiteness index was 1.4% higher than of hydrogen peroxide bleaching. Which is considered as acceptable whiteness index, so that the material would be ready for dyeing/printing. This acceptable degree of whiteness was decided in consultation with processing mills. The same results are shown in graphical form in Figure 1.

On the other hand when the tensile strength and fluidity values of hydrogen peroxide and peracetic acid bleached samples were examined, it was noticed that the tensile strength of PAA bleached sample was (2.5% warp direction; 3% weft direction) higher than hydrogen peroxide, also the fluidity values were changed from 1.4rhes (non bleached) to 2.0rhes in the case of PAA bleached sample and 2.5rhes in the case of hydrogen peroxide bleached samples. The fluidity value of PAA 2.0rhes shows the marginal degradation of cellulose than those of bleached sample by hydrogen peroxide. A report by Hickman W S and Andrianjafy H showed that the value of fluidity below 5rhes is considered acceptable for bleached fabric and Vaeck showed direct relationship between fluidity values and loss of tensile strength [15,16]. The results of tensile strength and fluidity are also exhibited in graphical form (Figures 2,3).

A big improvement in the absorbency (time required for the specular reflection of the water drop to disappear) (4.2 sec to 1.0 sec) were also observed in all cases of bleaching. The results of these figures are represented in Figure 4.

All the above results of CIE whiteness index, tensile strength, fluidity and absorbency obtained by PAA bleaching indicate that the main advantage of bleaching with PAA instead of peroxide is that a satisfactory degree of whiteness can be obtained at 60ºC in 40 minutes at neutral pH. This results in lower energy and water consumption in both during bleaching and rinsing of the fabric. Neutralization of the fabric after bleaching is not required, unlike bleaching with hydrogen peroxide, where large amount of alkali must be removed before dyeing. This is also much less damaging to the cotton fabric when PAA is used.

Conclusion
In this study PAA has been studied as an alternative to hydrogen peroxide for the bleaching of cotton. It has been demonstrated in this work that scoured cotton fabric can be bleached by PAA and it is possible to achieve an acceptable degree of whiteness in a shorter time than is required for hydrogen peroxide bleaching process.

Furthermore, bleaching can be carried out at 40ºC with neutral pH without producing any harmful chemicals.

PAA, as an industrial chemical is easily available and can be safely introduced to an existing process design.

References
1. Peters R H: Textile Chemistry, Elsevier Publ, 1967, Vol 2.
2. Easton B K: Ciba Geigy Rev, 1971, 3, 3.
3. Shenai V A: Technology of Bleaching and Mercerizing, Sevak Publications, New Dehli, 1991, pp10-60.
4. Cates D M and Cranor W H: Textile Res J, 1960, 30, 848.
5. Conzelmann F, Wurster P and Zahn A: Textil Praxis International, 1989, pp 644.
6. Schulz G: Textil Praxis International, 1990, pp 40.
7. Parch M et al: Fette Wachse, 1990, 77.
8. John Shore: Colourant & Auxiliaries, Hobbs The printers, Hampshire, UK,2002, Vol 2, pp 602-607.
9. Athur D Broadbent: Basic Principles of Textile Colouration, Society of Dyers & Colourists, UK, 2001, pp 132.
10. AATCC Technical Manual, Vol 75, Research Triangle Park: AATCC, 2000.
11. AATCC Technical Manual, Vol 66, Research Triangle Park: AATCC, 1991.
12. British Standard, BS EN ISO 13934-1: 1999.
13. Jones B M, Langlois G W and Sakaji R H: Environ Prog, 1985, 4, 252.
14. Rounsaville J and Rice R G: Ozone, Sci Eng, 1997, 18, 549.
15. Hickman W S and Andrianjafy H: J S D C, 1983, 99, 88.
16) Vaeck: J S D C,1966, 82, 374.

Note: For detailed version of this article please refer the print version of The Indian Textile Journal July 2010 issue.

Dr Farhan Khan
Textile Institute of Pakistan
R-525 block 19 F B Area Karachi 75950 Pakistan.
Email: farhankhan_tip@yahoo.com.