Oct 22, 2013 ... necessary for commercially acceptable consumer fabrics such as white and light
colored textile goods. Traditional textile bleaching of natural ...
HYDROPHOBIC BLEACH SYSTEMS AND TEXTILE PREPARATION: A DISCONTINUITY IN FABRIC CARE Jiping Wang and Nodie M. Washington The Procter & Gamble Company, Cincinnati, OH
Introduction In the textile processing of natural fibers, yarns, fabrics and their blends for commercial applications, a pretreatment or preparation step is typically required prior to the dyeing and finishing stages. This preparation step removes impurities and color bodies, imparted either naturally or by the spinning and weaving operations, from the fibers and fabrics to ensure uniform after treatments, good fabric absorbency and appearance. While textile treatments may include a number of varying processes and stages, the most common include: singeing - the removal of loose or miscellaneous fibers from the surface by burning with a flame; de-sizing - the removal of sizing agents, such as starches, via enzymatic soaking; scouring - the removal of greases, oils, waxes and fats by contact with a solution of sodium hydroxide at temperatures near boiling; mercerization - the application of high levels of sodium hydroxide in conjunction with stretching and pulling of the fabrics for increased fiber strength. An additional common pretreatment step involves bleaching to destroy naturally occurring color bodies. The bleaching step provides a uniform white appearance for consumer acceptable whites as well as a uniform color base for dyeing, printing and finishing. Thus, a highly successful bleaching step is necessary for commercially acceptable consumer fabrics such as white and light colored textile goods. Traditional textile bleaching of natural fibers and fabrics has involved the use of hydrogen peroxide. Hydrogen peroxide has gained wide acceptance due to its environmental friendliness and its flexibility for use in both hot and rapid or cold and long dwell bleaching processes. While hydrogen peroxide has gained wide spread acceptance in the textile industry, it is not a particularly effective bleaching agent. Hydrogen peroxide, as commercially supplied, is an extremely stable compound and as a result has only a slight bleaching effect. To overcome its weak activity, activation of the peroxide via the use of alkali, acid, uv irradiation, or organic activators is necessary with alkali being the most preferred. In addition, extremely high temperatures and/or extremely long bleaching times are also required in commercial processes. That is, temperatures in excess of 95° C are typically required or reaction times in excess of 20 hours in cold processing. This high temperature and/or long contact time process often results in significant fiber damage and fabric strength reduction which could be further deteriorated during dyeing and finishing. Organic activators have been attempted in textile industry bleaching systems with little success. Tetraacetylethylenediamine or TAED is a common hydrophilic bleach activator widely used in consumer laundry bleaching applications in Europe. Due to its surface inactivity and ineffectiveness under US conditions, TAED is not used in US laundry. Recently, TAED has also been explored in textile industry bleaching on cotton, wool and their blends1-3. However, while TAED provides some benefits, it gives little advantage over traditional hydrogen peroxide textile preparation. Additional hydrogen peroxide bleaching is still needed to provide superior fabric whiteness4. In addition, the poor water solubility of TAED limits its application in textile processing. Hydrophobic bleach activators, such as the sodium salt of nonanoyloxybenzene sulfonate (NOBS), have been employed in consumer laundry detergent applications such as Tide with Bleach to work in conjunction with hydrogen peroxide to provide activated bleaching in consumer laundering of gar226
ments5. Activated bleaching in consumer home laundry conditions allows effective cleaning of certain soils and stains under common US laundry conditions. Typical peroxide based textile bleaching systems involve hot batch, continuous or cold pad-batch processing. Hot batch and continuous peroxide bleaching both require the application of peroxide bleaching solutions at highly elevated temperatures (in excess of 95 °C). These elevated temperatures provide acceptable whiteness profiles on the treated fabrics and have a high throughput due to short reaction times, typically 60 minutes or less. However, hot temperature processing, while very effective, has the significant drawbacks of higher energy and cost due to the extreme temperatures required, expensive specialized processing equipment and importantly, increased fiber damage due to the aggressive conditions. In contrast to these drawbacks, cold batch processing involves the saturation of a textile with a peroxide based solution followed by long reaction times (more than 20 hours) at room temperature. While cold batch processing solves the aforementioned problems of fiber damage and elevated cost due to energy and specialized equipment, cold batch brings the significant disadvantage of an inability to achieve acceptable whiteness profiles, thereby limiting its use to dyed fabrics versus white fabrics. Low throughput due to the long reaction times reduces usefulness of cold pad batch. Accordingly, the need remains for an effective textile treatment process which will provide acceptable whiteness profiles and shorter reaction times in room temperature cold batch bleaching. Hydrophobic bleach systems, represented by the NOBS bleach system, overcome drawbacks from both hot and cold textile preparation. These hydrophobic systems reduce bleaching temperatures and/or shorten bleaching times, facilitating innovative textile preparation and enhancing fabric quality and care.
NOBS Chemistry Formation of an effective hydrophobic bleach from NOBS requires its reaction with hydrogen peroxide to create pernonanoic acid, the effective activated peroxide bleach. Under alkaline conditions, hydrogen peroxide reacts with NOBS to form pernonanoic acid, a low temperature bleaching agent. The reaction conditions must be controlled to 1) ensure maximum perhydrolysis of NOBS and 2) minimize side reactions. There are two side reactions which should be minimized. One is nucleophilic attack of pernonanoic acid anion on a yet unreacted NOBS molecule to form diacetylperoxide. The other is NOBS hydrolysis. Generation of Effective Pernonanoic Acid: 1). Ionization of hydrogen peroxide H2O2 + OH
HOO
-
HOO
-
+
H2O
2). Perhydrolysis of NOBS O C8H17
C O
SO3Na
+
NOBS O C8H17
C OOH
Pernonanoic Acid
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+
-
O
SO3Na
Side Reactions: Formation of diacylperoxide (DAP) O C8H17
O
C O
SO3Na
C8H17
+
C OO
-
NOBS O C8H17
O
C OO C
C8H17
DAP Hydrolysis of NOBS O C8H17
C O
O
-
OH SO3Na
C8H17
C OH
Nonanoic Acid
NOBS
Hydrophobic Bleach System in Textile Preparation As shown in the table below, the hydrophobic textile industry bleach system consists of a hydrophobic bleach activator (eg NOBS) with peroxide, and other additives. Caustic is used to assist the formation of the peracid and acts as a major desizing and scouring chemical. An anti-foaming agent may be needed for processes with high speed padding such as cold pad-batch and continuous to prevent excessive foam formation. It is not necessary for the hot batch process. Detergent and wetting agents are added to ensure removal of impurities such as wax, protein, sizing agents and to improve fabric water absorbency and uniformity of dyeing and finishing. Ingredients Hydrophobic Activator H2O2 NaOH Antifoaming Agent Detergent Wetting Agent
Functions To form hydrophobic peracid Bleach source Bleaching assistant, scouring and desizing May be needed for high speed padding Impurities Removal, better water absorbency Speed up wetting, uniform treatment
Application of a Hydrophobic Bleach System in Textile Preparation NOBS vs TAED It is observed under US laundry conditions that the hydrophobic peracid provides better cleaning than the hydrophilic peracid (peracetic) from bleaching systems such as TAED. This is also true in textile preparation processing. Due to their surface activity, hydrophobic bleach activators form the peracid on the surface of the fabric, increasing bleaching efficiency. Hydrophilic activators form peracid in solution and must then undergo a fabric solution interaction to generate bleaching action. As a result, hydrophobic bleaching agents provide superior bleaching, resulting in better fabric whiteness. A comparison of NOBS, TAED and H2O2 is provided. A 70 oC hot batch process was used on greige woven goods to evaluate a potential one-step preparation including bleaching, desizing and scouring together. As shown in the chart below, the NOBS system provides much better fabric bleaching than both TAED and traditional H2O2 228
bleaching. The NOBS system also provides improved fabric absorbency versus TAED. TAED’s poor water solubility likely limits its application in textile processing.
CIE Whiteness Index
Bleach Activators' Effect in One Step Preparation 90 80 70 60 50 40 30 20 10 0
H2O2 H2O2/NOBS H2O2/TAED
2
4
6
8
10
12
14
16
H2O2 (g/l)
Application in Cold Processing Cold pad-batch is gaining textile preparation market share due to small initial investment, and energy savings6, particularly in Latin America and Asia. However, there are two factors which often limit its broad application. First, considerable storage space is required to hold fabric, usually 16-24 hours, after the bleach solution has been applied. Second, traditional cold pad batch preparation is often not good enough to provide the high whiteness essential for white and light colored consumer goods. These two barriers for cold pad batch preparation are removed by hydrophobic bleach systems. As shown in the charts below, there are two major benefits from application of the NOBS system in cold pad-batch: Reduced process time. Shorten batch time from 24 hrs to 4 hrs with the same or better fabric whiteness and wettability. This will increase the operation flexibility/capability and significantly reduce the storage cost; this is a major limiter for cold pad-batch process. Better whiteness. Provide significantly whiter fabrics with less or the same storage time. This will raise fabric quality/specifications for this low cost process, broaden application areas and help to get market share from continuous and hot batch processes. The benefits are observed on a broad range of fabrics including greige cotton woven and knits, greige linen woven and linen blends, desized cotton woven, and more. NOBS System's Impact on Knits
NOBS System's Impact on Woven
75 Whiteness Index
Whiteness Index
80 75 70 65 60 Whiteness
75
65 60 55 50
Control at NOBS at 4 NOBS at 67.5
70
78.8
Whiteness
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Control at NOBS at 4 NOBS at 59.1
66.9
70.1
Application in Hot Processing Fiber damage is always a concern in hot textile bleaching. Textile fibers such as cotton are very sensitive to oxidation under the high pH and high temperature processing conditions. While fiber damage during preparation may not be big enough to cause significant fabric strength loss after preparation, it is often enhanced by dyeing and finishing, leading to severe fabric damage on consumer goods. This low temperature approach for the hydrophobic NOBS bleach system leads to lower fiber damage, measured by AATCC Test Method 82 (Cuen). The NOBS system provides competitive fabric whiteness at lower temperature. An example is shown in the chart below. Fabric whiteness data are shown at the top of the bars. Fiber Damage in Hot Batch Preparation
Fluidity (Cuen)*
W=77.6 8 7 6 5 4 3 2 1 0
W=67.9 W=75.8
NOBS system, 70C
H2O2, 70C
H2O2, 95C
* Lower fluidity indicates less fiber damage
Durable press (DP) finishes are broadly used in the textile and apparel industry to produce wrinkle resistant garments. The major drawback from DP finishing is fabric strength reduction caused by crosslinking and acidic cellulosic hydrolysis. Since the hydrophobic NOBS system reduces fiber damage in the preparation step, it provides stronger fabrics before application of the DP finish. As shown in the chart below, the fabric prepared by the hydrophobic NOBS system, then finished by the traditional DP finish (DMDHEU), has a strength retention at 69%. On the other hand, the fabric prepared by traditional high temperature preparation, then DP finished under the same conditions has significantly lower strength retention at 54%. This provides opportunities for the apparel industry to improve fabric strength on current wrinkle resistant garments and even broaden DP finishing to thin wovens such as 100% cotton shirting, where garment specifications on fabric strength are challenging. NOBS System + Durable Press Finish
Fabric Tensile Strength (kg)
25
69%
20
54%
15 10 5 0
Original
NOBS High T Preparation + DPPreparation + DP Finish Finish
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Conclusions Hydrophobic bleach systems such as NOBS are effective low temperature bleaching systems which overcome the drawbacks of the traditional H2O2 bleaching system. In hot processing, their temperature reduction capability improves fiber and fabric integrity resulting in enhanced fabric quality through finishing processes, including durable press. This not only provides benefits on cotton fabrics, but also offers unique solutions for those textile fibers (such as linen, linen blends, wool blends, etc) which cannot withstand traditional severe bleaching conditions. In cold processing, the hydrophobic bleach systems achieve levels of whiteness traditionally delivered from hot preparation and significantly shorten processing storage time- -leading to an increase in operation flexibility/capability and a broadening of fabrics prepared at room temperature.
References 1. 2. 3. 4. 5. 6.
Scarborough, S. Book of Papers, 1998 AATCC International Conference & Exhibition, 1998, p290. Cai, J. Y., Harrigan, F. J., Smith, S. M., Book of Papers, 1999 AATCC International Conference & Exhibition, 1999, p134. Milne, N. J., Watkins, J. M., US 5840667 Ahrens, H., Sauer, G., Redling, E., EP 0584710 Wang, J., Book of Papers, 1998 AATCC International Conference & Exhibition, 1998, p144. Dickinson, K., Hickman, W. S., JSDC, 101, 1985, p283.
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