Efficient and sustainable bleaching of cotton/spandex integrating UVA radiation and peroxide activator: A study focus on the degradation mechanism of natural pigments in fibres

2.1 Materials

Desized, unscoured and unbleached cotton/spandex (C/S) fabric (Specification: 97% cotton/3% spandex; density 290 g/m²), and 100% cotton (C) fabric (Density: 148 g/m²), were obtained from Zhejiang Tengma Textile Co., Ltd., China. TBCC (purity＞90%) was prepared through a condensation and substitution reaction based on a previous report (Cui et al., 2016). The structure of TBCC was confirmed by LC-MS and hydrogen nuclear magnetic resonance spectroscopy (¹H NMR) (Figure S1). Non-ionic penetrating agent (Industrial grade) was provided by Jiangsu Haian Petroleum Chemical Plant, China. Hydrogen peroxide (purity 30%), ethylenediamine tetraacetic acid disodium (EDTA-2Na) and sodium salts were in analytical grade. Morin (purity＞90%), nitrotetrazolium blue chloride (NBT, purity 98%), dimethyl sulphoxide (DMSO, purity ≥ 99.5%), benzoquinone (BQ, purity ≥ 99%) and sodium azide (SA, purity ≥ 99.5%), sodium oxalate (SO, purity ≥ 99.8%) were purchased from Aladdin. Benzenepentacarboxylic acid (BA, purity＞98%) was bought from TCI Chemical Industry Co. Ltd., China. Three direct dyes Direct Red 4BS, Direct Blue 15, Direct Yellow 12 with three representative colour hues (See chemical structures in Figure S2), were used for the dyeing of C/S fabric. Industrial neutral detergent for fabric soaping was obtained from Ningbo Runhe High-Tech Material Co., Ltd., China. Deionized water was used. All H₂O₂ mentioned refers to pure H₂O₂.

2.2 Fabric treatment

Fabrics were subject to padding (i), bleaching (ii) and washing & drying (iii) processes. For comparative study, four bleaching processes were formulated, i.e., UV-radiating bleaching (UV-Bl, a), steaming bleaching (St-Bl, b), cold pad-batch bleaching (CP-Bl, c), and conventional waterbath bleaching (Conv-Bl, d). Conv-Bl was performed by directly immersing fabric in bleaching bath without padding process. The processing parameters of Conv-Bl are listed in Table S1. UV-Bl is the major process explored in this study, while St-Bl, Conv-Bl and CP-Bl are only used to compare with UV-Bl. Ten gram of fabric was used for each process. Detailed processes are displayed in Fig. 1. Besides, two bleaching systems, i.e., NaOH/H₂O₂ (Table 1a) and TBCC/H₂O₂ (Table 1b) are incorporated to UV-Bl process. UV-Bl(ii) was carried out in a customized lamp-box installed with 6 UV lamps (See Figure S3). The power of each lamp was 15 W. The total radiation power can be adjusted to 30 W, 60 W or 90 W by a switch. Two kinds of lamps, i.e., UVA (TLD 15 W/03, Philips Co., Ltd.) and UVC (G15T8, SANKYO DENKI Co., Ltd.) were prepared in this research.

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Fig. 1

Four bleaching processes for the comparative study.

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2.3 Single factor test

Concentration of oxidants: The relationship between the concentration of oxidants and WI was explored using recipes of Group B in Table 1. C/S fabrics were bleached under 90 W UVA radiation for 1 h following procedure (a) in Fig. 1.

UV wavelength: Fabrics were first padded with bleaching solution of Group B-3,5,6, and then irradiated under UVA (365 nm) or UVC (254 nm) at power of 90 W for 1 h.

Power of UV lamp: Fabrics were first padded with bleaching solution of Group B-3,5,6, and then irradiated under UVA at power of 30 ∼ 90 W for 1 h.

UV exposure time: Fabrics were first padded with bleaching solution of Group B-5, and then irradiated under UVA at power of 90 W for 5 ∼ 120 min.

2.4 Dyeing experiment

The dyeing property of the bleached fabric was evaluated through dyeing with direct dyes with three-primary colours. Fabrics were immersed in the dyeing solution containing 0.5% owf dyes and 5 g/L Na₂SO₄ with a mass ratio (solution to fabric) of 25:1. The temperature of dyeing process started at 40 °C, raised up at a speed of 3 °C/min, and kept constant temperature of 90 °C for 45 min. Then the dyed fabrics were washed at 50 °C for 2 min followed by tap water for 2 min, and finally dried naturally.

2.5 Mechanism study

Degradation of morin in H₂O₂/TBCC system: A volume of 500 mL Morin solution (240 μmol/L) was prepared with the addition of NaOH (2 mmol/L) for dissolution. Then six identical mixtures containing 50 mL Morin solution, 50 mL TBCC solution (16 mmol/L), and 1 mL H₂O₂ solution (1 mo1/L) were prepared. Then, petri dishes holding as-prepared mixtures were loaded in the lamp-box under 90 W UVA or UVC radiation for different duration (0–30 min). In parallel, a series of conical flask holding the as-prepared mixtures were kept at constant temperatures without radiation for different duration. The Abs intensities of these mixtures were continuously detected by TU-1950 UV–Vis spectrophotometer (Beijing Persee General Instrument Co., Ltd.). To explore the role of TBCC in the morin degradation process, another set of experiment was carried out without TBCC for comparison.

Radical scavenging test: To explore valid radicals in degrading morin within TBCC/H₂O₂ solution, four kinds of radical scavengers, i.e., DMSO, BQ, SA and SO were used as the scavengers of HO•, O₂^–•, ¹O₂ and h⁺, respectively. (Cheng et al., 2022, Kaur et al., 2022). NBT was also included as the radicals’ probe (Dong et al., 2021). To verity the impact of radicals on WI improvement, DMSO and BQ were added into the actual bleaching solution to capture the radicals, respectively (Li et al., 2020, Wang et al., 2020). The fabric was immersed in a mixture of 1 g/L penetrating agent, 12.6 g/L TBCC, 4 g/L 30%H₂O₂, 3 g/L NaHCO₃ and 0–60 mL/L DMSO or 0–2.5 g/L BQ and bleached under UVA radiation for 45 min. The weight ratio of fabric to solution is 1:50.

LC-MS and MS: To demonstrate the completion of degradation reaction, LC-MS (Shimadzu, Japan) was used to identify the structural change of morin before and after degradation. The mobile phase consisted of 0.025% ammonia water (phase A) and acetonitrile (phase B) (A/B = 95%/5%∼5%/95%). Morin (6 mmol/L) was degraded in a homogeneous mixture containing 50 mmol/L TBCC, 60 mmol/L H₂O₂ and 2 mmol/L NaOH, followed by exposing the solution at UVA irradiation for 1 h. The degraded reaction solution was immediately freeze-dried at the end of radiation at −60◦C for 16 h to obtain the powder-like product. The degradation product was purified by column chromatography with the eluent consisting of methanol and methylene chloride. The molecular weight of the purified degraded products were detected by MS (Shimadzu, Japan) to speculate their chemical structures.

UV–Vis absorption spectroscopy: The concentration of morin was also monitored by UV–Vis absorption spectroscopy. The characteristic absorption peaks of morin existed at 275 nm, 317 nm and 410 nm, respectively (Sen and Yildiz 2017). The quantitative relationship between the absorbance at 410 nm and the concentration of morin solution was obtained in our previous study (Li et al., 2022). Based on this, the morin concentration at the initial time point (C₀) and certain time point (C) during the degradation process can be calculated. Furthermore, the degradation rate of morin was explored based on degradation kinetics calculated via the simplified Langmuir-Hinshelwood first-order kinetics model (See Eq. (1) (Cao et al., 2022)).

where K (min⁻¹) and t (min) are the rate constant and the reaction time, respectively.

Concentration of HO•: The HO• concentration was measured by FL labeling method according to a recent report (Si et al., 2014, Li et al., 2022). Benzenepentacarboxylic acid (BA) as a sensitive fluorescent probe was used to capture HO•. The measurement was carried out on F-2500 FL spectrophotometer (Hitachi, Japan). The parameters are as follows: band pass slit: 10.0 nm; excitation voltage of 700 V; Scan rate: 300 nm/min; EX wavelength: 307 nm).

To explore the effect of UVA radiation and bleaching time on the HO• concentration, a simulated bleaching solution containing 400 μmol/L BA, 200 μmol/L H₂O₂ and 200 μmol/L TBCC at a pH value of 10.5 adjusted by NaOH was prepared and subjected to the UVA irradiation (90 W) in the lamp-box for different duration (0–120 min). The FL intensity was taken by the end of treatment. Considering the test accuracy, fabric and surfactant were not used. As reference, another set of experiment went aside using the same condition in the absence of UVA radiation.

To further examine the influence of TBCC/H₂O₂ on the HO• concentration, a batch of simulated bleaching solutions at different TBCC/H₂O₂ concentrations (i.e., 100–800 μmol/L) were made with addition of 400 μmol/L BA. Then, above bleaching solution was subject to 90 W UVA radiation for 15 min. It is worth noting that the concentration of TBCC and H₂O₂ increased simultaneously according to 1:1 as the reaction molar ratio between TBCC and H₂O₂.

2.6 Measurement of fabric

Colour feature: CIE (L*, a*, b*), tristimulus values (X, Y and Z), colour depth (K/S) and whiteness index (WI) of fabric, were obtained from 110 reflectance spectrophotometer (Datacolor, USA) using D₆₅ illuminant at 10◦ standard observer. The yellowness index (YI) was calculated using Eq. (3),

Tensile strength: The YG028 universal material testing machine (Wenzhou Fangyuan, China) was adopted to evaluate the tensile strength of untreated (A-5) and bleached (B-5) fabrics based on ISO 2062: 2009. The two clamps held the fabric sample (50 × 300 mm²) and pulled it at a speed of 20 cm/min. The average results of each sample from five parallel tests in the both directions were reported.

ATR FT-IR: The ATR-FTIR spectra of untreated and bleached fabrics were recorded on a Nicolet iS50 ATR-FTIR spectrometer (Thermo Fisher Scientific Inc, USA) in the wavenumber range of 4000–500 cm⁻¹ at a resolution of 16 cm⁻¹.

XRD: The X-ray diffraction spectra of untreated and bleached fabrics were obtained from Ultima IV X-ray diffractometer (Rigaku Analytical Devices, Inc. Japan) at scattering angles ranging from 10° to 80°. The voltage and currency were 40 kV and 40 mA, respectively.

3.1 General evaluation of UVA-assisted bleaching based on TBCC/H₂O₂ system

3.1.2

3.1.2 Dyeability

Table S3 shows the lightness L*, a*[redness (+) and greenness (-)] and b* [yellowness (+) and blueness (-)] coordinates of untreated and bleached C/S fabrics. Untreated C/S fabric has small L* and large a*/b* values, and shows faint grey-yellowish appearance due to the abundantly existing impurities such as wax, natural pigments, seed shells, ash, etc. After UVA-assisted H₂O₂ bleaching, C/S fabric turns much whiter with an obvious increase in L* and decrease in a*/b* values. Further decrease in a*/b* values and less seed shells are witnessed on C/S fabric after UVA-assisted TBCC/H₂O₂ bleaching (See inserted photo). These colour features are consistent with the result in Table 2.

In the case of dyeing with three representative direct dyes, both blue and red dyes display higher saturation (C) and colour depth (K/S) on bleached than untreated C/S fabrics (Fig. 2), mainly due to the improved water absorbing capability of the fabric that facilitates a higher dye exhaustion. Although higher C of Direct Yellow 12 is occurred on bleached than untreated C/S fabrics, the K/S is conversely. The L* increased instead after dyed with yellow dyes. This may due to the high brightness of Direct Yellow 12 that could hardly cover the background grey-yellowish colour of the original fabric. In all, the coloration assessment above confirms the necessity for thorough UVA-assisted scouring/bleaching process using TBCC/H₂O₂ before the dyeing procedure.

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Fig. 2

Colour coordinates of dyed fabrics in 3D colour space and the corresponding fabric samples.

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3.2 Process factor analyses

It is confirmed that C/S fabric treated by UVA-assisted process using TBCC/H₂O₂ had qualified dyeability and tensile performance. This section attempts to further explore the impact of four significant factors (Concentration of 30%H₂O₂, UV band, UV Power, and radiation duration) on the bleaching effectiveness. With the increase of H₂O₂ concentration (Concentration of TBCC is determined according to Table 1), the WI displays a linear increase and reaches a maximum WI of 78 at 7.4 g/L (Fig. 3a). This indicates the pigment impurity was thoroughly reacted with the oxidants. However, to further increase the concentration of H₂O₂ and TBCC, the WI conversely decreased and the YI increased. A reason for this fact is as follows, the high concentration of TBCC was adsorbed on the fiber surface, which may generate steric hindrance hindering the penetration of TPA into the fiber interior (Chen et al., 2016). Thus, 7.4 g/L of H₂O₂ (30%) and 20 g/L of TBCC are recommended.

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Fig. 3

Single-factor analysis on the WI of C/S fabric: (a) Concentration of 30% H₂O₂, (b) UV band, (c) UV Power, and (d) radiation duration.

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As depicted in Fig. 3b, UVA enabled higher WI and lower YI of fabric compared with UVC after one hour radiation. Under UVC radiation, the WI of fabric continuously increased when higher concentration of oxidants was used, indicating that the pigment impurity on cotton still requires further oxidation. The optimal WI under UVA was obtained on sample B-5. There is a slight increase of WI at higher power (30 ∼ 90 W) of UVA radiation (Fig. 3c). Standing from the energy-saving perspective, the UVA power around 30 ∼ 60 W is able to achieve desirable WI on C/S fabric. Fast decolouration took place in the first 30 min (Fig. 3d), during which, the colour impurities especially those located on the surface or embedded in the shallow layer of cotton fibre are destroyed. During the UVA radiation from 30 to 90 min, WI and YI were stabilised around 78. The oxidants were further penetrated into fibre interior to further react with the rest of impurities. Obviously, time exceeding 90 min is too much for bleaching resulting in the increase of YI instead. This is due to the over-oxidation of the hydroxyl groups occurred on the cellulosic macromolecular of cotton fibre rising from condensed oxidant, thus generating significant numbers of carbonyl and carboxyl groups. These groups act as the precursors for the formation of chromophores which have strong and broadband optical absorption in the visible spectrum regions (Milanovic et al., 2020). In general, the concentration of oxidative couples and the processing time are two significant factors influencing the quality of fabric during UVA-assisted bleaching and should be paid attention to during practical processing.

3.3 Degradation mechanism study

3.3.1

3.3.1 Degradation rate of morin by altering temperature and UV wavelength

Contributed by the activated effect of TBCC, the degradation of morin almost reached equilibrium by the first 10 min (Fig. 4a). Thus, it is reasonable to apply non-linear fitting to the first-order kinetics model up to 10 min. The high R² close to 1 indicates the good fitness to the model. The larger K value means the faster degradation of morin. In conventional waterbath treatment, morin molecules degraded faster at higher temperature (indicated by K value) due to the accelerated physical and chemical reactions such as the perhydrolysis of TBCC for high concentration of TPA and its decomposition products ROS, the contact probability among morin molecules, TPA and ROS, and the oxidative degradation of morin by TPA and ROS. With the participation of UV light at ambient temperature (25 °C), the K value is much higher than that merely in waterbath. Such result demonstrates the accelerating function of UV on TBCC/H₂O₂ bleaching. It is also worth noting that UVC displayed lower efficiency than UVA, which is consistent with the effect of UVC on WI (see Fig. 3b). Additionally, the K value under UVA at 25 °C is close to that of 50 °C merely in waterbath treatment, in other words, to achieve a similar bleaching effect, the energy in heating up the waterbath and fabric are economized by introducing UVA radiation. For comparison, similar treatment processes without TBCC were carried out. Results are displayed in Fig. 4b. Obviously, the K value without TBCC is much smaller than that with TBCC. Thus combining the results in Fig. 4a and b, it proofs the rationality of the research design that integrates the TBCC and UVA irradiation into H₂O₂ bleaching process.

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Fig. 4

Degradation rate of morin in H₂O₂ oxidative system (a) with TBCC and (b) without TBCC.

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3.3.2

3.3.2 Determination of effective ROS for morin degradation

To further explore that bleaching mechanism, the role of different ROS in TBCC/H₂O₂ bleaching system and their contributions to the morin degradation, were explored through radical scavenging experiment. Four radical scavengers, i.e., dimethyl sulphoxide (DMSO), benzoquinone (BQ), sodium azide (SA) and sodium oxalate (SO) were used for HO•, O₂^–•, ¹O₂ and h⁺, respectively according to previous reports (Si et al., 2014, Cheng et al., 2022, Kaur et al., 2022). Upon the addition of these scavengers to four identical simulated solutions, the larger impact on degradation indicates the major role of the corresponding free radicals in the bleaching process. As seen in Fig. 5a, the addition of SO and SA show negligible impact on the degradation process compared with control sample. DMSO largely slowed down the degradation process followed by BQ. Such result demonstrates the HO• is the major free radical accounting for morin degradation followed by O₂^–•. There is almost no bleaching function of ¹O₂ and h⁺ in the solution. Similar conclusion was also drawn in the previous studies (Liu et al., 2018, Li et al., 2022).

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Fig. 5

(a) Impact of radical scavengers on morin degradation in UVA-assisted TBCC/H₂O₂ bleaching system; (b) Colouration reaction between NBT and O₂^–•; and impact of the concentration of (c) DMSO and (d) BQ on the WI, a* and b* of bleached fabrics.

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It has been confirmed that HO• is mainly from the decomposition of peroxy acid (Li et al., 2020). To further verify the source of O₂^–• in bleaching, NBT was added to the solution which enables colouration reaction with O₂^–• (Fig. 5b) (Dong et al., 2021). Without morin (Reference sample a), solution displayed colorless and transparent appearance compared with yellowish morin solution (Sample b)(See inserted photo). When the quantity of TBCC/H₂O₂ was set as 1:1, the depth of purple colour turned darker when UVA radiation prolonged to 40 min (Sample b,c,d). When 1/2 ratio of TBCC/H₂O₂ was used, the solution turned even darker than the corresponding solutions using 1/1 ratio of TBCC/H₂O₂, which indicates more O₂^–• was generated in the former case. Such phenomenon indicates that O₂^–• mainly originates from the decomposition of H₂O₂. Similar findings were also reported by other researchers (see reaction equation i,ii,iii) (Dannacber and Schlenker 1996, Gierer 1997, Wenhu et al., 2017). The function of HO• and O₂^–• in bleaching was further confirmed in practical bleaching process (Fig. 5c,d), in which the WI greatly reduced along with increasing reddish (a*) and yellowish (b*) intensity with the addition of their respective scavengers (DMSO and BQ). It is also found that the WI decreased drastically after the addition of DMSO due to the high oxidizing efficiency of HO•. However in the case of BQ, the WI decreasing pace is slower and the WI decreasing range is larger. This is probably because that O₂^–• owns lower reactivity and longer lifespan, and diffuses to a longer distance, than HO• (Nordberg and Arner 2001). In general, the analysis above confirms the UVA-assisted TBCC/H₂O₂ bleaching effect is dependable on HO• and O₂^–• radicals, and therein, HO• makes larger contribution than O₂^–• radicals.

(i)

H₂O₂ + HO₂^–→HO^–+HO₂•+HO•

(ii)

HO₂•→O₂^–•+ H⁺

(iii)

H₂O₂ + HO₂^–→H₂O + O₂^–•+ HO•

3.3.3

3.3.3 HO• concentration

Upon the confirmation of the HO• as the major bleaching ROS, the concentration of HO• under different irradiation time and using different concentration of TBCC was explored in this section. As seen in Fig. 6a and b, the FL intensity representing the concentration of HO• continuously increases towards 8 k units under UVA radiation as the time approaches to 120 min, which is around 8 times over that without UVA radiation. This reveals the function of UVA to initiate the generation of HO• in TBCC/H₂O₂ bleaching system. By using higher concentration of TBCC, the concentration of HO• displays an increase almost along a linearship relation (Fig. 6c). The results above demonstrate that the UVA irradiation and participation of TBCC as the precursor of TPA are two significant factors to generate HO•.

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Fig. 6

Concentration of HO• in TBCC/H₂O₂ system (a) with and (b) without UVA radiation, and (c) using different TBCC concentrations.

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Fig. 6

Concentration of HO• in TBCC/H₂O₂ system (a) with and (b) without UVA radiation, and (c) using different TBCC concentrations.

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3.3.4

3.3.4 LC-MS and MS analyses

To gain further understanding of degradation mechanism, the degrading pathway of morin in UVA-assisted TBCC/H₂O₂ system was explored by LC-MS and MS. In Figure S6, the molecular ion peak at m/z 303.0 (M + H) was identified as the evidence of undegraded morin by LC-MS method at a retention time of 0.901 min. In terms of the morin after radiation treatment (Figure S7), the characteristic peak at m/z 303.0 disappeared, demonstrating that the morin was completely degraded. The retention times are 0.222 min and 0.784 min, representing the molecular ion peak of perhydrolyzed side-products of TBCC (caprolactam, m/z 114.0 and 4-(triethylammoniomethyl) benzoic acid, m/z 236.1), and unreacted TBCC (m/z 331.1), respectively. In other words, unreacted TBCC and its two perhydrolyzed side-products in the bleaching solution are detected by LC-MS. However, there is not sufficient information about the degraded morin segments, probability due to the much stronger signal intensity of TBCC and its perhydrolyzed side-products over that of the degraded segments of morin. Thus, column chromatography was adopted for precise isolation of each product in the mixture before MS analysis.

As depicted in Fig. 7, two major residual segments from morin after isolation were detected by MS, which are 2,4,6-trihydroxybenzoic acid (MW: 170.12) and 2,4,-dihydroxybenzoic acid (MW: 154.12). Thus, the degrading routine of morin under UVA-assisted TBCC/H₂O₂ system is proposed as follows (See the inserted reaction formulas). First, HO• radicals generated by TPA and UVA irradiation attack the C-ring of morin through 1,4-conjugate addition reaction to generate semi-acetal intermediate (1). Second, the instable semi-acetal intermediate turns into tricarbonyl intermediate through ring-open reaction (2). Finally, the electron-deficient carbonyl is attacked by HO• radicals through nuclear addition, hydrolysis and decarboxylation reactions (3) (Ilunga and Meijboom 2016). Thus, P₁ and P₂ are generated (Bingwa et al., 2018, Ilunga et al., 2018).

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Fig. 7

MS (positive ion mode) of degradation product and speculated degradation pathway of the morin.

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3.4 Sustainability evaluation

The advantage of UVA-assisted bleaching process using TBCC/H₂O₂ in sustainable textile industry is evaluated with respect of water, energy and time consumptions referencing to the WI enhancement of fabric. Four processes including UV-Bl, CP-Bl, St-Bl and Conv-Bl (Fig. 1) were involved and compared. A recipe matrix accompanied with corresponding energy/water consumption and resultant WI enhancement, are displayed in Table 5. The detailed calculation methods of energy and water consumption are stated in Supporting Information.

Following the treatment recipes in the matrix, an evident trend for WI improvement is discovered. UV-Bl process exerts equivalent whiteness upgrade on the fabrics to those treated by Conv-Bl, and also displays superior WI increment over St-Bl and CP-Bl recipes, which verifies the application prospect of UV-Bl as an efficient bleaching process. An obvious energy reduction occurs on UV-Bl recipes compared with St-Bl and Conv-Bl processes when similar WI enhancement is achieved. A general comparison of the energy consumptions in different recipes is described in the pie chart (Figure S8). The main energy consumptions for St-Bl and Conv-Bl processes rise from the evaporation of the water on fabric (taking up 54.5% of the total) and the heating of the bleaching bath (taking up 89.6% of the total), respectively. In terms of UV-Bl, energy is mainly consumes by UV lamps rather than those used for phase change in St-Bl and heating bath in Conv-Bl processes, thus displaying the energy-saving advantages.

Approximate WI enhancement is obtained from UV-Bl-3 and Conv-Bl-2 processes, however 95.6% energy is saved in the front procedure. Compared the UV-Bl-2 and UV-Bl-3 processes, a significant reduction in energy consumption from 2.7 × 10⁴ KJ to 0.9 × 10⁴ KJ takes place when 30 W UVA power is applied instead of 90 W, however resulting in only 4.3% decline in ΔWI. In other words, UV-Bl-3 is preferable standing from the energy-saving perspective if there is no strong requirement on the whiteness. Considering the bleaching effectiveness, 46.5% WI enhancement is achieved using 30 W UVA radiation and TBCC/H₂O₂ system (UV-Bl-3), which is similar to that (47.3%) using 90 W UVA radiation and NaOH/H₂O₂ system (UV-Bl-1). Excitedly, the energy consumption in the frontier process is only 1/3 of that in the latter process, which confirms the significant energy-saving merit rising from the activation of TBCC for H₂O₂. Theoretically, there is no energy consumption during CP-Bl process as it is an ambient-temperature procedure. Although 40.4% WI enhancement is achieved in CP-Bl process slightly lower than UV-Bl-4 process, the time required in CP-Bl is 96 times over that of UV-Bl-4, resulting in a low processing efficiency during the continuous mass production. The pH of the process is highly related to the water consumption due to the pH neutralization, in other words, the near-neutral process benefits the water saving. Within the design matrix, UV-Bl-3 enables large WI enhancement with less energy, water and time consumption.