Abstract:
This study discusses the application of ozone to bleach raw cotton fabric using both
laboratory scale set up and pilot plant. The quality of ozone bleached fabric was determined
in terms of CIE whiteness, absorbency, degree of polymerisation (DP), copper number (CN)
and tendering factor (TF) with regard to the conventionally bleached fabric (reference
material). The application of ozonation process was also extended for color stripping of dyed
cotton fabric at laboratory scale. The efficiency of ozone stripping process was measured in
terms of lightness (L*), lightness difference (∆L*) and total color difference (∆E*) with
respect to reference (conventionally stripped fabric samples).
In case of laboratory scale application of ozone bleaching process, the maximum CIE
whiteness (60-62) and best quality of bleached fabric was obtained at an ozone dose of 6 g/h,
pH 5 and exposure time of 45 min. Strong acidic environment (pH 2-3) and exposure time
longer than 45 min though marginally improved the degree of whiteness but deteriorated the
quality of the fabric as indicated by higher copper number (0.19) and TF (0.94). Statistical
analysis of experimental data confirmed that process parameters (ozone dose, pH and
treatment time) significantly affected the efficiency of ozone bleaching process and pH had
the greatest effect on whiteness and DP of bleached fabric followed in turn by ozone dose
and treatment time. The dyeing quality of ozone bleached fabric in terms of colour difference
(∆L*, ∆a*, ∆b* and ∆E*) and wash fastness properties was almost similar to the reference.
The ozone bleaching waterbath was also reused several times to bleach multiple lots of raw
cotton fabric and results demonstrated that CIE whiteness (60) remained stable even at 20 th
reuse of same waterbath. This approach reduced the pollution load of effluent and saved
water and chemicals.
Since the efficiency of ozone bleaching process at pilot scale was not in parallel with its
laboratory scale performance, various additives were used to enhance process efficiency.
The best results in terms of whiteness (63.79), absorbency (10 sec) and strength of bleached
fabric were achieved with an addition of 2 g/l of surfactant at optimal process conditions
(ozone dose of 50 g/h, pH 5 and ozone treatment time of 45 min). However, other additives
(peracetic acid and H2O2) did not show promising results.
Dyed ozone bleached fabric samples were a bit lighter (ΔL* > 0.4) than reference. The
values of ΔL*, Δa* and Δb* showed minor color difference between ozone bleached fabric
and reference. However, total color difference values (ΔE* = 0.13 – 1.06) were within
acceptable limit (ΔE*<1). Wash fastness properties of ozone bleached fabric were identical
to the reference.
The whiteness of ozone bleached fabric, however, did not stand firm with increasing storage
time due to the presence of aldehyde groups and ozone residues. Among various treatments
performed (hot washing at 50 ◦C, rinsing at 80 ◦C, reductive treatment) to avoid the decrease
in degree of whiteness of ozone bleached fabric, washing with sodium borohydride was
proved as one of the best treatments for color stability.
Ozone color stripping efficiency was a function of pH, ozone dose and treatment time and
maximum color stripping was achieved at an ozone dose of 10 g/h, pH 5 and treatment time
of 45 min. The stripping efficiency decreased as the % owf of dyed fabric increased from 2%
to 4% which implied that deceolorization of dyed fabric with higher initial dye concentration
required higher ozone dose and/or longer exposure time. The color stripping efficiency of
ozone for fabric samples dyed at different shades was almost similar to the reference
(conventional stripping method).
The ozone bleaching process was proved to be environmentally friendly because it decreases
water consumption (no rinsing and neutralizing treatment), it is less energy intensive
(applicable at ambient temperature), and reduces effluent load (involves no chemicals).