Knit Fabric Mercerisation through the Use of High-Concentration NaOH in a Scouring and Bleaching Bath using an Exhaustion Method

The combined scouring, bleaching and mercerising of single jersey cotton knitted fabrics through the use of high-concentration NaOH was evaluated in this study. The eff ect of that new combined process was compared, contrasted and analysed with pristine scoured, bleached fabric and conventional bleached fabric. Surface morphology, structural analysis and the colour strength of dyed fabrics were examined using a scanning electron microscope (SEM), X-ray diff raction (XRD) and spectrophotometric analysis. The barium activity number and bursting strength of treated fabric was evaluated and analysed in accordance with the relevant AATCC test standards. It was found that a sample treated through combined scouring, bleaching and mercerising demonstrated a higher level of smoothness and lustre than typical scoured and bleached fabric. The treated sample exhibited decreased crystallinity, as well as the transformation of the crystalline phase in a similar manner. The barium activity number, bursting strength and improved K/S value were also in line with the separately mercerised fabric. The above described properties of the combined scoured, bleached and mercerised fabric are sound evidence of the eff ectiveness of the process.


Introduction
Mercerisation is the best-known method for enhancing the physical and dyeing properties of cotton bre.Mercerisation changes the surface morphology, ne structure (i.e.crystallinity), crystal size, etc. of cotton bre [1,2].It converts the cellulose chain from cellulose I to cellulose II [3][4][5].A signi cant improvement in dye a nity, colour strength, lustre, tensile strength and the smoothness of cotton fabric also occurred due to mercerisation [1,6,7].e degree of change depends on di erent parameters, such as processing time, processing temperature, tension, the concentration of caustic soda, etc. [8].e mercerisation of knitted fabric was considered impractical due to stretching in the process itself, but is essential for enhancing the properties of knitted fabric [9].Today, di erent manufacturers design machines for knit fabric mercerisation that does not harm the dimensional stability of fabric.Both open width and tubular fabric can be mercerised with newly invented machinery without damaging the quality of fabric.Highly concentrated NaOH solution (14% or higher) has been used extensively to modify cellulosic bre for a long period of time.Such a modi cation of cellulosic fabric improves the dye receptivity of cotton and reduces the shrinkage of cotton fabric [1].e development of slack mercerisation resolves many problems associated with knit fabric mercerisation [10].Fabric used to be treated with high-concentration caustic soda without tension to achieve excellent physical and dyeing properties [11,12].Mercerisation at high temperature is now a well-established method [13].e penetration of caustic soda into the bre structure is more rapid [14], which ensures more uniform treatment but less swelling [8]. is in turn enhances lustre without a ecting the strength of the bre.On the other hand, high temperatures and high-concentration caustic soda give fabric a plastic-like nature, which results in signicant improvements in properties [15].A compatibility chart of dyes and chemicals shows the compatibility of caustic soda with hydrogen peroxide.Several studies have reported combined scouring and mercerising at high temperatures for di erent methods.Processing, however, is not deemed sustainable due to its limited application.Taking into account all of the above facts, this study hypothesised that modi ed slack mercerisation without tension through the use of high temperatures, caustic soda and hydrogen peroxide would prove to be a simple and novel approach.As a part of our study, the properties of fabric pre-treated with combined scouring, bleaching and mercerising were contracted and compared with mercerised fabric and with individual scoured, bleached fabric.
e changes in surface morphology, ne structure and physical properties were assessed to validate the e ectiveness of combined scouring, bleaching and mercerising.

Materials
Single jersey cotton knitted fabric of 160 g/m 2 was purchased from Fakhruddin Textile Mills Ltd., Bangladesh.All the chemicals (NaOH, H 2 O 2 , sequestering agent, anti-creasing agent, stabiliser and detergent) were purchased from the Aziz Group, Bangladesh.

Methodology
ree samples were prepared for the study.Sample 1 (S1) was a conventional scoured and bleached single jersey knitted fabric, while sample 2 (S2) was fabric treated by conventional scouring and bleaching followed by caustic mercerisation at room temperature and sample 3 (S3) was single jersey cotton knitted fabric treated with combined scouring, bleaching and mercerising.All the parameters of common processes were kept constant for all three samples.Figure 1 illustrates the process curve of the preparation of all samples.Conventional scouring and bleaching of S1 and S2 was performed using 0.6 g/l detergent, 0.5 g/l sequestering agent, 0.5 g/l anti-creasing agent, 2 g/l NaOH, 3 g/l H 2 O 2 and 0.4 g/l stabiliser.S2 was later treated with 20% NaOH solution for mercerisation purposes.S3 was prepared to bring out the e ect of mercerisation at a high concentration (16 g/l) of NaOH during scouring and bleaching, which is referred to as combined scouring, bleaching and mercerising.For further study, S1, S2, and S3 were bound together and dyed with a vinyl-sulfone reactive dye applying an exhaustion method in the same bath.To achieve precise results, dyeing was performed using three di erent shade percentages, i.e. 1%, 3% and 5%.NaOH in a Scouring and Bleaching Bath using an Exhaustion Method Tekstilec, 2017, 60(4), 324-330

Characterisations
Changes of surface morphology were assessed using a JSM-6510 scanning electron microscope (JEOL Ltd.).Accelerating voltage of 20 KV and magni cations of 2000x and 3000x were used during scanning microscopy.Fine structural changes were analysed using an MDI JADE-6 X-ray di ractometer.Crystallinity was determined using the ratio of integrated crystallite scattering intensity to total scattering intensity ranging from 10 to 36°. e amount of crystallinity and amorphous fraction were determined using synthesised curves to di ractograms obtained from the experiment through successive approximation.Lorentzian distributions, which represent crystalline component peaks, the amorphous component and contribution due to the air scattering of the X-ray beam, were calculated using a programmable calculator.
e degree of mercerisation and barium activity number were measured using the AATCC 89-1998 test method.Samples were taken from all three fabrics and cut into small pieces.is was followed by the treatment of 1 g of each fabric type with 30 ml of 0.25 barium hydroxide solution in a 100 ml conical ask for two hours.Ten ml from each sample solution was then titrated against 0.1 N HCI.To calculate the result, a blank solution of barium hydroxide was also titrated.e result was obtained using the following equation: , where: BAN represents the barium activity number; T 1 represents the titration reading for untreated fabric; T 2 represents the titration reading for mercerised fabric; and T 3 represents the titration reading for un-mercerised fabric.Before testing bursting strength, all fabrics were conditioned and le to achieve a relaxed state.A er full relaxation, bursting strength was measured using a bursting strength tester.e test was carried out 10 times for each sample, and the average value recorded and expressed in kg/cm 2 .e K/S value of dyed fabric was determined using a spectrophotometer (Datacolor-650).Initially, the re ectance percentage of dyed fabric was measured.e K/S value was then calculated applying the Kubelka-Munk theory.e K/S value can be obtained using the following equation: 3 Results and discussion

Changes in surface morphology
It is a well-known fact that the natural deep wrinkles from the surface of cotton bre tend to vanish due to caustic mercerisation.e cell wall of thebre becomes thicker, depending on maturity, while the natural twisted ribbon-like structure tends to change and form a round-like structure.Porous or granular phenomena appeared on the surface of the bre.e gures clearly show that the bres demonstrate a round structure instead of a twisted ribbon-like structure.A signi cant number of the natural deep wrinkles were removed from the surface.In all cases, the bre structure appears to have a smoother surface than the previous one.A smoother surface allowed for more regular re ectance, which resulted in the more lustrous surface of the fabric.e lustre of the combined scoured, bleached and mercerised fabric (S3) is far better than the conventional scoured and bleached fabric (S1).ere were no signi cant di erences in terms of lustre between the combined scoured, bleached and mercerised fabric (S3) and separately mercerised fabric (S2).  of S3. e results obtained from the XRD are summarised in Table 1.Among the three tested samples, the crystallinity of S2 and S3 decreased signi cantly, with S3 demonstrating a more signi cant decrease in crystallinity than S2. is can be explained by di erences in temperature.A high mercerisation temperature decreased the viscosity of NaOH solution and enhanced the di usion property by swelling the highly ordered area of bre and decreasing the hydrophobic nature of natural impurities.However, the di erence in crystallinity between the two mercerised fabrics is immaterial. is is due to the lower concentration of caustic soda in S3 than in S2.Table shows that both mercerised fabrics (S2 and S3) exhibit the crystalline phase of cellulose II, while the amount of cellulose II is higher in S2. e extent of the decrease in crystallinity and the conversion of cellulose I to cellulose II are strong evidence of the e ectiveness of combined scouring, bleaching and mercerising.

Barium activity number
e most widely used method to determine the extent of mercerisation is the barium activity number.Table 2 shows the barium activity number of both S2 and S3.

Bursting strength
Table 3 shows the bursting strength of the three tested samples.e bursting strength of both S2 and S3 is higher than S1. e increase in strength can be attributed to the alleviation of internal stresses and the de-twisting of the normal ribbon-like structure ofbre due to the swelling process.e bursting strength of S2 is higher than that of S3. e reason lies in the lower concentration of caustic soda and tension during mercerisation.A higher degree of orientation can be achieved if the fabric is kept under tension during mercerisation.High temperature is also somewhat correlated to strength, as high-concentration alkali can cause harm to the bre at high temperatures.It is a well-known fact that mercerisation increases the strength of cotton fabric.In both processes (S2 and S3), the strength of fabric increased signi cantly, clear evidence of the e ectiveness of combined scouring, bleaching and mercerisation.

Change in colour strength
Figure 6 shows the K/S value of all three samples dyed with vinyl-sulfone reactive dye at di erent concentrations.S2 and S3 showed a higher K/S value than S1 for all shade percentages.An X-ray di raction diagram and scanning electron microscopy show that signi cant changes occurred in the structure of the fabrics due to the treatment.ese structural changes had a remarkable e ect on the colour strength of the dyed cotton fabric.As the crystallinity of both mercerised fabrics decreased, dye molecules easily penetrated the bre.A comparison of the two mercerised fabrics (S2 and S3) indicate that the separately mercerised fabric (S2) demonstrated a higher K/S value because of its higher crystallinity relative to the other fabric.is can be attributed to the crystallite size and crystalline phase, i.e. the orientation of bre structure.S2 has a higher crystallite size and higher amount of cellulose II in the structure of bre than S3.Although there is little di erence between S2 and S3, both demonstrated a higher K/S value than the un-mercerised fabric (S1).ere was strong evidence that mercerisation can also be achieved by using high-concentration caustic soda during scouring and bleaching.

Conclusion
Cotton single jersey knitted fabric was mercerised using a new process, and the e ect of that process on several physical properties of treated fabric successfully validated.e e ectiveness of combined scouring, bleaching and mercerisation was proven through surface morphology, ne structure (crystallinity, crystallite size and crystalline phase) and barium activity number.e K/S value of all three fabrics dyed in di erent concentrations was also evidence of the e ect of the combined process.An SEM surface imagery analysis of all the samples indicated the e ect of the new method on smoothness and lustre due to the round-like structure.
e decrease in crystallinity, the conversion of cellulose I to cellulose II and crystallite size con rm the e ectiveness of the process.e barium activity number of the two mercerised fabrics falls within the range of standard values.e improvement in the bursting strength and K/S value of the dyed fabrics prove that combined scouring, bleaching and mercerising enhance physical properties to the same extent as separately mercerised fabric.Further research is required regarding this sustainable approach to textile processing.Additional analysis and validation of changes in the dimensions and density of the treated fabric can be enhanced during advanced studies.

Figure 1 :
Figure 1: Process curves for: a) scouring and bleaching, b) scouring, bleaching and separate mercerising, c) combined scouring, bleaching and mercerising, and d) dyeing of all types , where: K represents absorbance; S represents scattering; and R represents re ectance [%].

A
er the cotton bre was treated with high-concentration NaOH, it formed a cellulose-soda-water compound.During the destruction of the aforementioned, cellulose converted from cellulose I to cellulose II. is conversion of cellulose depended entirely on the treatment condition.Most of the converted cellulose failed to return to its original position, resulting in an increase in the amorphous region of the cotton bre and a decrease in crystallinity.Figure 5 presents an XRD diagram of conventional scoured and bleached fabric (S1), separately mercerised fabric (S2) and combined scoured, bleached and mercerised fabric (S3).It is evident that S1 demonstrates normal crystallinity, while S2 demonstrates a signi cantly reduced crystallinity, which may have occurred due to the penetration of high-concentration caustic soda into the highly oriented crystalline region.e same phenomenon occurred in the case

Figure 6 :
Figure 6: K-S value of samples

Table 2 :
Barium activity number of S2 and S3 Although the value of the barium activity number is lower, it is still within the standard range (130-150).islower value can be explained by the higher temperature and lower concentration of caustic soda.
A comparison of S2 and S3 indicated that combined scouring, bleaching and mercerising results in a lower barium activity number.

Table 1 :
Summary of information from XRD diagram

Table 3 :
Bursting strength of di erent fabrics