Bleaching of Kenaf and Cornhusk Fibers - American Chemical Society

Feb 8, 2007 - UniVersity of NebraskasLincoln, 234 HE Building, Lincoln, Nebraska ... alternative sources for natural cellulose fibers.1-6 Recently, hi...
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Bleaching of Kenaf and Cornhusk Fibers Abdus Salam,† Narendra Reddy,† and Yiqi Yang*,†,‡ Department of Textiles, Clothing & Design and Department of Biological Systems Engineering, UniVersity of NebraskasLincoln, 234 HE Building, Lincoln, Nebraska 68583-0802

Kenaf and cornhusk fibers have been bleached to CIE whiteness indexes of 66 and 74, respectively, without using any optical brighteners. A delignification process prior to bleaching has been used to partially remove the lignin from the fibers without affecting other fiber properties. Bleaching variables such as the concentration of hydrogen peroxide, time, temperature, and pH have been optimized for both the kenaf and cornhusk fibers. The effects of various bleaching parameters on the whiteness index and the breaking tenacity of the fibers have been reported. The whiteness of kenaf fibers reported here is high compared to the whiteness achieved by other bleaching processes developed earlier. Introduction Finding alternative sources for natural and synthetic fibers is gaining importance lately mainly due to the several limitations associated with the availability and cost of the currently used natural and synthetic fibers. For example, synthetic fibers are from nonrenewable petroleum sources that are becoming scarce and expensive. Similarly, traditional fiber crops such as cotton and linen need natural resources such as land, water, and energy. Compared to the natural and synthetic fibers in current use, lignocellulosic agricultural byproducts are cheap, abundant, and annually renewable resources that are being considered as alternative sources for natural cellulose fibers.1-6 Recently, highquality natural cellulose fibers with properties suitable for textile applications have been produced from cornhusk, cornstalks, and rice straw.1-5 Efforts are also being made to understand and improve the properties of the fibers obtained from several other annually renewable resources and make them suitable for various applications.6-9 Similarly, kenaf has been considered as an alternative crop to linen and jute and several researchers are working to improve the properties of kenaf and make it useful for textile applications.10-13 The advantages of kenaf compared to the conventional fiber crops such as jute, hemp, and flax are the short growing time, easy adaptability to climatic conditions, and relatively less use of herbicides and pesticides.12 Kenaf, cornhusk, cornstalk, rice, switchgrass, and other fibers obtained from lignocellulosic sources are all multicellular fibers with several single cells held together by lignin and other binding materials in the form of a fiber bundle. These fibers typically have lignin contents ranging from 8 to 15% compared to 1-2% in cotton and linen.1-6,10,12 Although lignin is necessary to hold the single cells together and provide a useful fiber bundle, lignin adversely affects several fiber properties. The presence of lignin produces a natural color to the fiber, the color ranging from light yellow in cornhusk fibers to dark brown in kenaf and rice straw fibers. The natural color in the fibers is difficult to remove, and it is not possible to bleach the lignocellulosic fibers to high whiteness using the methods used to bleach the common cellulose fibers. In addition, the presence of lignin makes the hand of the fibers to be harsh and also accelerates photodegradation of the fibers when exposed to UV light.14,15 * To whom correspondence should be addressed. Tel.: (402) 4725197. Fax: (402) 472-0640. E-mail: [email protected]. † Department of Textiles, Clothing & Design. ‡ Department of Biological Systems Engineering.

Kenaf and cornhusk fibers represent two alternative fiber sources that have different compositions and structures.1-6,10-12 Although these two fibers are reported to have good properties and are said to be processable as textile fibers, their bleachability as textile fibers has not been fully understood. The only two reports on bleaching kenaf fibers have not been able to obtain high whiteness values and the fibers also have high loss in breaking tenacity.12,13 The best whiteness reported for kenaf fibers is a CIE whiteness index (WI) of 46, which is low compared to the WI of about 80 for bleached cotton.12,13 No reports are available on the bleaching of cornhusk fibers. Several approaches are used to chemically modify and/or partially delignify lignocellulosic fibers to improve their properties, bleach the fibers to high whiteness, and make them suitable for textile applications. Such modifications have mainly been used for jute and include sulfonation and graft polymerization of various monomers.14-19 Sodium sulfite used during sulfonation reacts with the aromatic rings of lignin and makes lignin water soluble.20 Jute fibers modified using these techniques have been bleached to relatively high whiteness (WI of about 74) without affecting the fiber properties.16,18 In this paper, we have partially delignified kenaf and cornhusk fibers and studied the effect of various bleaching parameters on the WI and breaking tenacity of the fibers. Optimum conditions for bleaching both the kenaf and cornhusk fibers to high whiteness values with minimum damage to the breaking tenacity of the fibers have been developed. Materials and Methods Materials. Kenaf fibers used in this study were obtained from Greene Natural Fibers, Snow Hill, NC. The fibers are reportedly grown in North Carolina and are mechanically processed. The properties of the kenaf fibers used in this study are given in Table 1. Cornhusk fibers were produced in our laboratory using the methods described earlier.2,3 The chemicals used in the study such as 30% hydrogen peroxide (Mallinckrodt), sodium silicate (Titri Star), sodium sulfite (Merck), soda ash (Merck), monoethanolamine (TCI America), and ethylenediamine (EDA) (J.T. Baker) were reagent-grade chemicals purchased from VWR International, Bristol, CT. Methods. A. Delignification. Kenaf fibers were delignified by treating with 15% sodium sulfite, 0.5% MEA (monoethanolamine), and 4% soda ash, all percentages based on the weight of the fiber, at 140 °C for 3 h using a high-pressure canister. Similarly, cornhusk fibers were delignified using 9% sodium

10.1021/ie061371c CCC: $37.00 © 2007 American Chemical Society Published on Web 02/08/2007

Ind. Eng. Chem. Res., Vol. 46, No. 5, 2007 1453 Table 1. Properties of Cornhusk and Kenaf Fibers before and after Delignification Compared with Properties of Raw Cotton Fibers parameter lignin, % whiteness index fineness, denier breaking tenacity, g/denier breaking elongation, %

cornhusk raw delignified 6 -112 110 ( 14 1.8 ( 0.2

3.5 -91 70 ( 7 1.7 ( 0.1

raw

kenaf raw delignified cottona

13 -121 50 ( 4 2.6 ( 0.4

7 -66 24 ( 6 2.6 ( 0.2

0-1 -40 3-5 2.7-3.5

11.9 ( 1.1 11.4 ( 0.5 1.8 ( 0.3 1.3 ( 0.4 6.0-9.0

a The range of lignin, fineness, breaking tenacity, and breaking elongation data for cotton are from ref 31. The whiteness index of cotton is from this research.

sulfite, 0.3% MEA, and 4% soda ash at 150 °C for 2 h. A liquor to fiber ratio of 10:1 was used for the delignification studies. These conditions were chosen based on our previous experiences of delignifying jute fibers to obtain optimum removal of lignin without affecting the properties of the fibers.18 The amount of lignin in the fibers was determined according to ASTM Method D 1106-96. B. Bleaching. The raw and modified kenaf and cornhusk fibers were bleached in a launderometer. Bleaching was carried out using hydrogen peroxide in the concentration range of 1.23.0%, with 5% sodium silicate; all percentages were based on the weight of the fiber. American Association of Textile Chemists and Colorists (AATCC) standard detergent without optical brightener (2% weight of the fiber) was used as a wetting agent. Treatment variables that were studied to obtain the optimized conditions were pH 5-12, temperature from 60 to 100 °C, time from 0.5 to 2.5 h, and liquor ratio from 5 to 1 to 25 to 1. The delignification conditions and ranges of variables for bleaching were selected based on our previous experiences in bleaching jute fibers.18 C. Testing. The raw, delignified, and bleached fibers were tested for their CIE whiteness index (WI) and tensile properties. WI was tested using a Hunter Ultrascan XE spectrophotometer using a D 65/10° observer and 1 in. circular viewing area. Samples to be measured were placed in a 2.5 in. glass sample cup. Three replications were taken for each sample, and the average is reported. The standard deviation between the three readings was less than 1.0, and therefore only the average is reported. The tensile properties of the fibers in terms of breaking tenacity and breaking elongation were determined on an Instron tensile tester using a gauge length of 1 in. and a crosshead speed of 18 mm/min. About 50 fibers were tested for each condition studied, and the average and standard error of the mean are reported. D. Morphological Structure. A Hitachi Model S3000N scanning electron microscope (SEM) was used to observe the changes in the surface of the raw, delignified, and bleached fibers. Results and Discussion Effect of Delignification. The effect of delignification on the properties of the cornhusk and kenaf fibers is shown in Table 1. About 40 and 45% lignin is removed from the cornhusk and kenaf fibers, respectively, by the delignification method adopted here. Delignification results in finer and less yellow fibers for both the kenaf and cornhusk fibers. The fineness of the cornhusk fibers increases by about 30% and that of the kenaf fibers increases by about 45% after delignification. The whiteness of the cornhusk and kenaf fibers increases by 21 and 55 absolute units after delignification compared to the respective values

Figure 1. (a) Effect of time on the whiteness index of raw and delignified cornhusk and kenaf fibers. The effect of time was studied using a peroxide concentration of 2.7% at 100 °C in a pH 10 solution with liquor to fiber ratio of 15 to 1 for the kenaf fibers and using a peroxide concentration of 2.1% at 90 °C in a pH 10 solution with liquor to fiber ratio of 10 to 1 for the cornhusk fibers with 5% sodium silicate. (b) Effect of time on the breaking tenacity retention of raw and delignified cornhusk and kenaf fibers. The effect of time was studied using a peroxide concentration of 2.7% at 100 °C in a pH 10 solution with liquor to fiber ratio of 15 to 1 for the kenaf fibers and using a peroxide concentration of 2.1% at 90 °C in a pH 10 solution with liquor to fiber ratio of 10 to 1 for the cornhusk fibers with 5% sodium silicate.

before delignification. The breaking tenacity and breaking elongation of either the cornhusk or the kenaf fibers do not change appreciably after delignification. The raw kenaf fibers have more than twice the amount of lignin in the raw cornhusk fibers and are therefore slightly more yellow. However, the kenaf fibers are much finer, have higher breaking tenacity, but have lower breaking elongation than the cornhusk fibers. The higher whiteness of the delignified kenaf fibers compared to the delignified cornhusk fibers despite having higher lignin content should mainly be due to the presence of different types of lignins in the fibers. Kenaf fibers are reported to have various types of lignins, and during delignification the lignins contributing to the color are removed, resulting in relatively white fibers.21,22 Also, the cornhusk fibers are obtained by alkaline extraction, whereas the kenaf fibers used in this study are obtained by mechanical separation. Lignin in the cornhusk fibers turns yellow under alkaline conditions, and the yellow color contributes more to the reduction in whiteness when the WI of the fibers is measured. Effect of Time. The effect of increasing time on the WI of the four groups of fibers studied is shown in Figure 1a. The delignified cornhusk and kenaf fibers have positive WI (27 and 35, respectively) even at the lowest time of bleaching as seen from Figure 1a, whereas the raw cornhusk and kenaf fibers have negative WI (-89 and -63, respectively). Increasing the time

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of bleaching continuously improves the whiteness of all the fibers up to 1.5 h, but the whiteness decreases at higher bleaching times. The raw cornhusk and kenaf fibers have negative WI (-85 and -9, respectively) even after bleaching for 2.5 h. The highest WI for the delignified cornhusk (67) and kenaf fibers (58) are obtained when the bleaching is done for 1.5 and 2 h, respectively, using a peroxide concentration of 2.7%, at 100 °C in a pH 10 solution with liquor to fiber ratio of 15 to 1 for the kenaf fibers and a peroxide concentration of 2.1% at 90 °C in a pH 10 solution with liquor to fiber ratio of 10 to 1 for the cornhusk fibers. In the traditional cellulose bleaching process for fibers with low or no lignin contents, increasing time increases the whiteness of the fibers gradually and then the whiteness stabilizes without further change at long durations of bleaching.23,24 However, the kenaf and cornhusk fibers show a decrease in whiteness when bleached for 2.5 h compared to the respective whiteness of the fibers at 2 h. Such reduction in whiteness on prolonged bleaching times has also been seen during bleaching of jute.18 This reduction in whiteness at 2.5 h of bleaching for the kenaf and cornhusk fibers is most likely due to the presence of lignin. Lignin probably forms some oxidized products that turn yellow during bleaching, but the yellowness is not evident during the initial stages of bleaching since the increase in whiteness of the cellulose masks the slight yellowing of the samples. The higher reduction of whiteness for the raw fibers which have higher lignin contents compared to the delignified fibers corroborates the assumption on the contribution of lignin to the yellowing during bleaching at longer times. Increasing the time of bleaching continuously decreases the breaking tenacity retention of the fibers, as seen from Figure 1b. At the lowest time of bleaching (0.5 h), the raw and delignified cornhusk fibers have similar retention in breaking tenacity, but the breaking tenacity retention is higher for the delignified fibers than for the raw fibers at longer bleaching times. This difference in breaking tenacity retention also exists for kenaf. The lowest breaking tenacity retention occurs at a bleaching time of 2.5 h for all the fibers. At the bleaching time of 2 h, which gives the highest WI with a peroxide concentration of 2.7%, at 100 °C in a pH 10 solution with liquor to fiber ratio of 15 to 1 for the kenaf fibers, the strength retention is 74 and 86% for the raw and delignified fibers, respectively. Similarly, the breaking tenacity retention at 1.5 h when the highest WI is obtained is 50 and 62% for the raw and delignified cornhusk fibers, respectively, with a peroxide concentration of 2.1% at 90 °C in a pH 10 solution with liquor to fiber ratio of 10 to 1 for the cornhusk fibers. The higher breaking tenacity retention of the delignified fibers compared to the raw fibers suggests that lignin affects the breaking tenacity of the raw fibers more than the delignified fibers. Effect of Temperature. Temperature of bleaching plays a critical role with respect to the whiteness of the fibers, as seen from Figure 2a. Except for the delignified kenaf fibers, all other fibers have negative WI at the lowest bleaching temperature (60 °C) studied. The raw kenaf fibers have a positive WI above a temperature of 80 °C, whereas the raw cornhusk fibers have negative WI even after bleaching at 100 °C. However, the delignified cornhusk and kenaf fibers have positive WI at all temperatures except for the delignified cornhusk fibers bleached at 60 °C. The highest whiteness (59) is achieved at a temperature of 100 °C for the delignified cornhusk fibers and at 90 °C for the delignified kenaf fibers (WI of 66) with a peroxide concentration of 2.7% for 2 h at pH 11 with liquor to fiber ratio of 15 to 1 for the kenaf fibers and using a peroxide

Figure 2. (a) Effect of temperature on the whiteness index of raw and delignified cornhusk and kenaf fibers. The effect of temperature was studied using a peroxide concentration of 2.7% for 2 h at pH 11 with liquor to fiber ratio of 15 to 1 for the kenaf fibers and using a peroxide concentration of 2.1% for 2 h at pH 11 with liquor to fiber ratio of 15 to 1 for the cornhusk fibers and 5% sodium silicate. (b) Effect of temperature on the breaking tenacity retention of raw and delignified cornhusk and kenaf fibers. The effect of temperature was studied using a peroxide concentration of 2.7% for 2 h at pH 11 with liquor to fiber ratio of 15 to 1 for the kenaf fibers and using a peroxide concentration of 2.1% for 2 h at pH 11 with liquor to fiber ratio of 15 to 1 for the cornhusk fibers and 5% sodium silicate.

concentration of 2.1% for 2 h at pH 11 with liquor to fiber ratio of 10 to 1 for the cornhusk fibers. Increasing the temperature decreases the breaking tenacity retention for the kenaf fibers, whereas there is an optimum temperature range of 80-90 °C where the cornhusk fibers have relatively high breaking tenacity retention as seen from Figure 2b. The delignified fibers have higher breaking tenacity retention and the cornhusk fibers have a lower breaking tenacity retention than the kenaf fibers at all temperatures. The raw (58%) and delignified (66%) cornhusk fibers have relatively high breaking tenacity retention at 90 °C, whereas the raw and delignified kenaf fibers have a relatively high breaking tenacity retention of about 75 and 83%, respectively, when bleached at 90 °C. However, the highest WI for the delignified cornhusk fibers (59) is obtained at a temperature of 100 °C with breaking tenacity retention of about 44% and the highest WI for the delignified kenaf fibers (66) is obtained at 90 °C with breaking tenacity retention of about 83%. The rate of bleaching is reported by previous researchers to increase with increase in temperature, and there is very slow bleaching action below 80 °C.25 However, higher temperatures tend to make hydrogen peroxide unstable and also result in greater degradation of cellulose.25 High temperatures cause rapid decomposition of the peroxide without providing enough time

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Figure 3. (a) Effect of liquor to fiber ratio on the whiteness index of raw and delignified cornhusk and kenaf fibers. The effect of liquor to fiber ratio was studied using a peroxide concentration of 2.7% at 90 °C for 2 h at pH 11 for the kenaf fibers and using a peroxide concentration of 2.1% at 90 °C for 2 h at pH 10 for the cornhusk fibers with 5% sodium silicate. (b) Effect of increasing liquor to fiber ratio on the breaking tenacity retention of raw and delignified cornhusk and kenaf fibers. The effect of liquor to fiber ratio was studied using a peroxide concentration of 2.7% at 90 °C for 2 h at pH 11 for the kenaf fibers and using a peroxide concentration of 2.1% at 90 °C for 2 h at pH 10 for the cornhusk fibers with 5% sodium silicate.

for bleaching, and therefore the bleaching effect is decreased at very high temperatures. The rapid decomposition of peroxide at higher temperatures causes more damage to the fibers, and therefore the fibers have lower breaking tenacity retention at higher temperatures. Effect of Liquor Ratio. The concentration of hydrogen peroxide and other chemicals used to study the effect of liquor ratio were based on the weight of the fibers. Therefore, increasing the liquor ratio means decreasing the chemical concentration in the bleaching bath. Increasing the liquor ratio from 5 to 1 to 15 to 1 increases the whiteness of all the fibers, but the WI of all the samples decreases upon further increase of the liquor ratio, as seen from Figure 3a. At a liquor ratio of 5 to 1, both the delignified kenaf and delignified cornhusk fibers have positive WI whereas the raw fibers have considerably negative WI. In fact, the raw cornhusk fibers have negative WI at all the liquor ratios studied. Increasing the liquor ratio increases the breaking tenacity retention for all the fibers, as seen from Figure 3b. The highest tenacity retention for all the fibers is at a liquor ratio of 25 to 1. The delignified fibers have higher breaking tenacity retention

Figure 4. (a) Effect of pH on the whiteness index of raw and delignified cornhusk and kenaf fibers. The effect of pH was studied using a peroxide concentration of 2.7% at 100 °C for 2 h with liquor to fiber ratio of 15 to 1 for the kenaf fibers and using a peroxide concentration of 2.1% at 90 °C for 2 h with liquor to fiber ratio of 15 to 1 for the cornhusk fibers and 5% sodium silicate. (b) Effect of increasing pH on the breaking tenacity retention of raw and delignified cornhusk and kenaf fibers. The effect of pH was studied using a peroxide concentration of 2.7% at 100 °C for 2 h with liquor to fiber ratio of 15 to 1 for the kenaf fibers and using a peroxide concentration of 2.1% at 90 °C for 2 h with liquor to fiber ratio of 15 to 1 for the cornhusk fibers and 5% sodium silicate.

than the raw fibers and the kenaf fibers have higher strength retention than the cornhusk fibers at all liquor ratios studied. At low liquor ratios, there is not enough free liquid for even bleaching of the fibers and the chemical concentration is also high. This results in lower whiteness and higher strength loss as seen from parts a and b, respectively, of Figure 3. Increasing the liquor ratio means diluting the concentration of hydrogen peroxide, and therefore there is lower whiteness but higher tenacity retention at high liquor ratios. A liquor ratio between 10 to 1 and 15 to 1 provides the most optimum concentration of peroxide for obtaining white fibers with moderate breaking tenacity loss. Effect of pH. The effect of increasing pH on the whiteness of the four groups of fibers is depicted in Figure 4a. Increasing the pH increases the WI for the fibers up to pH 11, except for the delignified kenaf fibers. A pH between 10 and 11 is the most optimum for the fibers, except for the raw kenaf fibers. The raw kenaf fibers have a WI of 29 at pH 12, whereas the raw cornhusk fibers are still yellow and have a WI of -62. The highest WI obtained for the delignified cornhusk fibers is 54 at pH 11, whereas the delignified kenaf fibers have the highest whiteness index value of 58 at pH 10. The bleaching power and the stability of hydrogen peroxide are dependent on the pH.25 Hydrogen peroxide is stable at pH

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1-3 but is least stable in high alkaline pH, i.e., between 11.5 and 13.25 If the pH is too low, no bleaching takes place, but if the pH is too high, the bleaching liquor becomes very unstable with low bleaching power. Therefore, a pH between 10 and 11 is desirable for bleaching since it is possible to achieve good whiteness without substantially affecting other fiber properties. However, the raw kenaf fibers are an exception to this pH range under the conditions used to study the pH effect. This deviation from the optimum for the raw kenaf fibers is probably due to the high lignin content in the fibers that may only be removed at high pH. The cornhusk fibers have been treated at high pH and high temperature during fiber extraction, and therefore the lignin removal is easier for the cornhusk fibers. Figure 4b shows the effect of increasing the bleaching pH on the breaking tenacity retention of cornhusk and kenaf fibers both before and after delignification. Even at the lowest pH used, both the raw and delignified cornhusk fibers have only about 50% breaking tenacity retention (48 and 46%, respectively). The breaking tenacity retention for the cornhusk fibers is relatively higher at pH 10 and 11, especially for the delignified fibers. The lowest retention in breaking tenacity for the cornhusk fibers is experienced at pH 12 for both the raw and delignified cornhusk fibers. Both the raw and delignified kenaf fibers have higher breaking tenacity retention than the cornhusk fibers at all the pHs studied. However, the delignified kenaf fibers have higher breaking tenacity retention than the raw fibers, except at pH 12. Initially, at pH 8, the raw kenaf fibers have breaking tenacity retention of 64% compared to breaking tenacity retention of 73% for the delignified kenaf fibers. At the practical hydrogen peroxide bleaching pH 10 and 11, the raw kenaf fibers have breaking tenacity retention of 74 and 75%, respectively, whereas the delignified kenaf fibers have higher breaking tenacity retention of 86 and 83%, respectively. However, the breaking tenacity retention of the raw and delignified kenaf fibers decreases to 75 and 71%, respectively, at pH 12. In the traditional cellulose bleaching for fibers with no or low lignin contents, the breaking tenacity loss is minimum at a pH between 9 and 10.26,27 However, the fibers studied here have higher strength loss at pH 8 and 9 compared to the breaking tenacity loss at pH 10 and 11. Such differences in the breaking tenacity retention of lignocellulosic fibers at pH 10 and 11 have also been previously observed for jute.18 The reasons for the higher breaking tenacity retention of lignocellulosic fibers at pH 10 and 11 are unclear at this time, but are probably due to the damage of lignin and hemicellulose which serve as binding agents at lower pH. Since the cornhusk fibers have low lignin contents compared to kenaf fibers, the damage due to the lignin affects the breaking tenacity loss of the cornhusk fibers to a greater extent than for the kenaf fibers. Effect of Hydrogen Peroxide Concentration. Figure 5a shows the changes in the WI of the raw and delignified fibers with increasing hydrogen peroxide concentration studied using pH 10 solutions at 100 °C for 1 h. The corresponding parameters for the cornhusk fibers are pH 10 solution at 90 °C for 90 min. Increasing the concentration of hydrogen peroxide without changing the other parameters increases the whiteness of all the fibers, but the increase in whiteness becomes less pronounced. At the lowest concentration of peroxide used, both the raw cornhusk and kenaf fibers have a WI of -104 whereas the delignified cornhusk and kenaf fibers have a positive WI of 33. Both the delignified cornhusk and kenaf fibers have a positive WI after bleaching even at the lowest concentration of hydrogen peroxide used. At the highest amounts of hydrogen peroxide used, the raw cornhusk and kenaf fibers are still yellow

Figure 5. (a) Effect of hydrogen peroxide concentration on the whiteness index of raw and delignified cornhusk and kenaf fibers. The effect of peroxide was studied in a pH 10 solution at 100 °C for 1 h with liquor to fiber ratio of 15 to 1 for the kenaf fibers and in a pH 10 solution at 90 °C for 90 min with liquor to fiber ratio of 10 to 1 for the cornhusk fibers and 5% sodium silicate. (b) Effect of hydrogen peroxide concentration on the breaking tenacity retention of raw and delignified cornhusk and kenaf fibers. The effect of peroxide was studied in a pH 10 solution at 100 °C for 1 h with liquor to fiber ratio of 15 to 1 and in a pH 10 solution at 90 °C for 90 min with liquor to fiber ratio of 10 to 1 for the cornhusk fibers and 5% sodium silicate.

with a WI of -77 and -33, respectively. However, the delignified cornhusk and kenaf fibers become white with WI of 74 and 56, respectively. Although the WI of kenaf fibers is still low, the cornhusk fibers have excellent whiteness compared to bleached cotton, which typically has a WI of about 80. The amount of peroxide to be used to bleach a particular fiber depends on the composition of the fiber and also on the other bleaching variables such as time, temperature, pH, and liquor to fiber ratios used. The differences in the level of whiteness between the raw and delignified cornhusk and kenaf fibers should be due to the inherent yellowness of the samples, the amount and properties of lignin in the samples, and also the variation in the amount of peroxide used. For example, although the cornhusk fibers have lower lignin contents than the kenaf fibers, the cornhusk fibers are more yellow than the kenaf fibers at the same concentration of peroxide used. The lower whiteness of the cornhusk fibers should be mainly due to the yellow color of the fibers formed during fiber extraction compared to the natural brownish color of kenaf fibers. The yellow color of the cornhusk fibers probably contributes more to the reduction in whiteness than the brownish color. One of the negative effects of hydrogen peroxide bleaching is the breaking tenacity loss in cellulose fibers. As seen from Figure 5b, increasing the concentration of peroxide decreases the breaking tenacity retention of the fibers. At the lowest

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Figure 6. SEM pictures of raw kenaf fibers (a) showing surface deposits that are probably composed of hemicellulose, lignin, and other materials. Delignification removes most of the surface substances, resulting in fibers with relatively smooth surface (b). Bleaching further removes any surface substances, resulting in fibers that show cellulose fibrils on the surface (c).

concentration of hydrogen peroxide used, the raw and delignified cornhusk fibers have breaking tenacity retention of about 73 and 81% whereas the breaking tenacity retention of the raw and delignified kenaf fibers has relatively less change compared to the respective unbleached controls. However, further increase in concentration of peroxide decreases the breaking tenacity retention for all the fibers, and the loss in breaking tenacity is higher for the cornhusk fibers than the kenaf fibers. At the highest concentration of peroxide, the raw and delignified cornhusk fibers have breaking tenacity retention of about 33 and 44%, respectively. Similarly, the breaking tenacity retention for the raw and delignified kenaf fibers are 77 and 83%, respectively, at the highest concentration of peroxide used. Increasing the hydrogen peroxide concentration increases the formation of free radicals that cleave the β-glucosidic linkages in cellulose, leading to a decrease in the degree of polymerization of cellulose and therefore lower breaking tenacity retention.28-31 Previous reports on bleaching kenaf fibers using hydrogen peroxide at milder conditions report a higher loss in breaking tenacity.13 In that report, kenaf fibers bleached at the mildest (4% peroxide, 75 °C, 2 h) and strongest conditions (10% peroxide, 85 °C, 3 h) experienced breaking tenacity loss of 26 and 29%, respectively, but the whiteness obtained by the bleaching conditions adopted here has not been reported. In another report, kenaf has been bleached using hydrogen peroxide with various other additives to help remove lignin and obtain

white fibers.12 The highest whiteness obtained in that research is 46 (CIE WI), much lower than that achieved here, even using a much higher concentration (5%) of hydrogen peroxide.12 Assuming the breaking tenacity of the raw fibers used in that study to be 2.6 g/denier as used in this study, the breaking tenacity loss at the highest WI obtained in that research was about 45%. The breaking tenacity loss for kenaf fibers in this study is only 12% at the highest WI (66) obtained. In addition, the bleaching reported by Wang and Ramaswamy has been carried out using detergents such as AATCC soap and also commercial detergents such as Gain and Tide. These detergents contain UV absorbers (fluorescent brightening agents) which give abnormally high whiteness values. If similar detergents containing UV absorbers are used for kenaf and cornhusk fibers, the WI jumps to 89 and 100, respectively, compared to a WI of 66 and 74 when detergents without UV absorbers are used during bleaching. Similarly, the WI of standard Test Fabric bleached 400 cotton fabric increases to about 113 from 84 when treated with 2% (by weight of the fabrics) detergents with UV absorbers in water at 90 °C for 90 min without any other chemicals. Morphological Structure. Parts a, b, and c, respectively, of Figure 6show the surface features of the raw, delignified, and bleached kenaf fibers. The raw fibers have a layer of surface deposits that enclose the cellulose fibers inside. This outer layer is mostly composed of hemicellulose and lignin. After delignification most of the surface substances are removed and the

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fibers have a clean surface as seen from Figure 6b. Bleaching leads to further removal of the surface substances, and therefore the bleached fibers are clean to an extent that the cellulose fibrils in the fibers are also visible as seen from Figure 6c. Conclusions The kenaf and cornhusk fiber bleaching method reported here is capable of producing white fibers. The whiteness of kenaf fibers achieved in this research (CIE WI of 66) is high compared to the highest WI (46) previously reported. However, since lignin is the major contributor to the color of the fibers, it is necessary to delignify the fibers to achieve high whiteness values. The delignification process adopted in this research is capable of removing about 50% lignin and makes the fibers finer without affecting the tensile properties of the fibers. A peroxide concentration of 2.7% at 90 °C for 2 h in a pH 11 solution has been found to provide the highest whiteness for the kenaf fibers. The corresponding values for the cornhusk fibers are a peroxide concentration of 2.4% at 90 °C for 90 min in a pH 10 solution. The method of delignification and bleaching developed here should be suitable for other lignocellulosic fibers also. Acknowledgment We thank the Consortium for Plant Biotechnology Research (DOE Prime Agreement No. DE-FG36-02GO12026) and the Procter & Gamble Company for their financial support. Funding from an AATCC student research grant, the USDA Hatch Act, and the Agricultural Research Division at the University of NebraskasLincoln is also thankfully acknowledged. Literature Cited (1) Reddy, N.; Yang, Y. Biofibers from agricultural byproducts for industrial applications. Trends Biotechnol. 2005, 23, 22. (2) Reddy, N.; Yang, Y. Properties and potential applications of natural cellulose fibers obtained from cornhusk. Green Chem. 2005, 7, 190. (3) Reddy, N.; Yang, Y. New long natural cellulosic fibers from cornhusk: structure and properties. AATCC ReV. 2005, 5 (7), 24. (4) Reddy, N.; Yang, Y. Structure and properties of high quality natural cellulose fibers from cornstalks. Polymer 2005, 46, 5494. (5) Reddy, N.; Yang, Y. Properties of high quality long natural cellulose fibers from rice straw. J. Agric. Food Chem. 2006, 54, 8077. (6) Karst, D.; Yang, Y. Molecular modeling study of the resistance of PLA to hydrolysis based on the blending of PLLA and PDLA. Polymer 2006, 47, 4845. (7) Karst, D.; Yang, Y.; Genzo, T. An explanation of increased hydrolysis of the β-(1,4)-glycosidic linkages of grafted cellulose using molecular modeling. Polymer 2006, 47, 6464. (8) Yang, Y.; Huda, S. Dyeing conditions and their effects on mechanical properties of polylactide fabric. AATCC ReV. 2003, 3 (8), 56. (9) Yang, Y.; Huda, S. Comparison of disperse dye exhaustion, color yield, and colorfastness between polylactide and poly(ethylene terephthalate). J. Appl. Polym. Sci. 2003, 90, 3285. (10) Ramaswamy, G. N.; Ruff, C. G.; Boyd, C. R. Effect of Bacterial and Chemical Retting on Kenaf Fiber Quality. Text. Res. J. 1994, 64 (5), 305.

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ReceiVed for reView October 25, 2006 ReVised manuscript receiVed December 22, 2006 Accepted January 15, 2007 IE061371C