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H3PO4–NANO2 mediated oxidation on the structure and properties of cellulose fibers
Accepted Manuscript Title: Influence of HNO3 /H3 PO4 –NANO2 mediated oxidation on the structure and properties of cellulose fibers Author: Yunhui Xu Xin Liu Xuelan Liu Jiulong Tan Hongling Zhu PII: DOI: Reference:
S0144-8617(14)00499-8 http://dx.doi.org/doi:10.1016/j.carbpol.2014.05.029 CARP 8896
To appear in: Received date: Revised date: Accepted date:
4-2-2014 26-4-2014 3-5-2014
Please cite this article as: Xu, Y., Liu, X., Liu, X., Tan, J., & Zhu, H.,Influence of HNO3 /H3 PO4 ndashNANO2 mediated oxidation on the structure and properties of cellulose fibers, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.05.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Influence of HNO3/H3PO4–NANO2 mediated oxidation on the structure and properties of cellulose fibers
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Yunhui Xu a,*, Xin Liu b, Xuelan Liu c, Jiulong Tan a, Hongling Zhu a
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The bamboo pulp cellulose fiber was oxidized with HNO3/H3PO4–NaNO2 mixture to obtain oxidized cellulose
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containing different levels of carboxyl content and with high yields. The effects of HNO3/H3PO4–NaNO2 mediated
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oxidation on structure and properties of the fiber were investigated. The results showed that an increase in carboxyl
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content and weight loss of oxidized fibers appeared with increasing oxidation time. Compared with the original
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cellulose, the oxidized fibers had lower crystallinity (29–40%) and thermal stability. The patterns of
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X-ray diffraction and other testing methods revealed that the oxidation mostly occurred at C6 primary hydroxyl
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groups of cellulose. Moreover, an oxidized fiber with 94.14–98.59% of high yields and 1.13–3.56% of carboxyl
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content was obtained in the range of oxidation time from 15 to 60 min, while its mechanical properties did not
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change significantly. This work presented some detailed information about structure-property correlations of
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oxidized bamboo pulp cellulose fibers and was useful in planning applications of these products.
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Keywords: Bamboo pulp cellulose; Oxidation of C6 position; Monocarboxy cellulose; Crystalline structure; NMR
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1. Introduction
College of Light-Textile Engineering and Art, Anhui Agricultural University, Hefei, Anhui 230036, China College of Natural Sciences, Anhui Agricultural University, Hefei, Anhui 230036, China c Key Laboratory of Zoonoses of Anhui Province, Anhui Agricultural University, Hefei, Anhui 230036, China
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C-NMR,
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Abstract
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The unrenewable resources of petroleum and coal become increasingly exhausted, so there is a growing attention
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to utilize natural biopolymers. In this regard, fiber plants consisting of cellulose, hemicelluloses and lignin acquire
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enormous significance as rich biomass and chemical feedstock since they are containing many functional groups
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suitable to chemical derivatization (Khullar et al., 2008). Bamboo is a woody herbaceous plant and consists of many
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vascular bundles and xylem. Cellulose pulp prepared from bamboo is around 57–66%, and especially α-cellulose
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gets about 26–43% that is comparable with that of softwoods and hardwoods (He et al., 2008; Shanmughave, 2003).
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Compared with other counterpart timber species, bamboo is an abundant renewable natural resource capable of fast
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growth (3–5 years), high productivity and holocellulose content (64–70%), and easy propagation, which is not like * Corresponding author. Tel.: +86 551 6578 6459; fax: +86 551 6578 6331. E-mail address: [email protected] (Y.H. Xu).
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wood to suffer insect infestation. Bamboo pulp cellulose fiber is a novel kind of green and environment-friendly
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fiber that has been successfully industrialized in China. At present, the regenerated bamboo cellulose fiber is
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produced in the form of dissolving pulp of bamboo cellulose by using viscose or Lyocell spinning procedure.
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However, it is very different from traditional viscose fiber in the raw material and process. Bamboo pulp fiber has
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markedly different properties from cotton and wood fibers, such as lustrous looking, soft and cool feelings, good
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drapability, high water and perspiration adsorption, abrasion resistance, good dyeability, anti-ultraviolet, and
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excellent thermal conductivity (Ma et al., 2011; Shen et al., 2004). Bamboo pulp cellulose fibers have already
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attained an increasingly important position in the clothing and medical textile industry.
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Chemical modification of polysaccharides is an important approach for preparation of new materials with
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improved performance and value-addition (Liu & Sun, 2008; Schurz, 1999). In principle, chemical modification of
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native and regenerated cellulose fibers by selective oxidation reactions of primary and secondary hydroxyl groups
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of the pyranose ring in cellulose offers an attractive means for acquisition of new properties by introduction of
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aldehyde and carboxyl active groups to a polymer surface (Gert et al., 2005; Montanari et al., 2005). The partial
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oxidation renders cellulose bioabsorbable in human, on such way introduced functionalities further broadens the
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potential applications of these biodegradable polymers, and it has been applied in medical devices such as
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absorbable adhesion barriers and hemostatic agents, scaffold in tissue engineering, drug delivery matrix, biosensors,
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etc., while the hemostatic properties and absorption of oxycellulose can be adjusted by the degree of oxidation (Hon,
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1996).
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Oxidized cellulose containing carboxyl groups is commercially available by reacting cellulose with NO2 or N2O4
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in the gaseous form or as a solution in an appropriate organic solvent (Ashton, 1968; Bertocchi et al., 1995; Wu et
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al., 2012). Other oxidants usually applied to produce monocarboxy cellulose include HNO3, HNO3–NaNO2,
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HNO3/H2SO4–NaNO2, H3PO4–NaNO2 and H3PO4–NaNO2–NaNO3, etc (Bertocchi et al., 1995; Gert et al., 1995;
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Painter et al., 1985; Wanleg, 1956). The reaction with HNO3 alone or a mixture of H3PO4–NaNO2 requires heating
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at high temperature, these cause extensive hydrolysis of cellulose and low yield of oxycellulose. The HNO3–NaNO2
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can prepare oxycellulose with 14–18% carboxyl content, but requires a long oxidation time. The mediated
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oxidation of HNO3/H2SO4–NaNO2 is quicker compared to that with HNO3–NaNO2 alone and achieves the oxidized
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product with up to ~21% carboxyl content. However, the yield of oxycellulose was decreased to about 20–30% due
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to further hydrolysis by strong acid of H2SO4. Oxidation of primary hydroxyl groups of celluloses using a mixture
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of HNO3/H3PO4–NaNO2 as an oxidant has become one of the interesting routes to introduce carbonyl and carboxyl -2-
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functionalities into polysaccharides (Kumar & Yang, 2002; Painter et al., 1985; Wang et al., 2009). The advantages
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of HNO3/H3PO4–NaNO2 mediated oxidation of polysaccharides are the followings: high selectivity and yield, just
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modest degradation of celluloses through out the oxidation process, and this oxidation proceeds under aqueous mild
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conditions around room temperature (Kumar & Yang, 2002; Saito et al., 2006). On the other hand, the
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HNO3/H3PO4–NaNO2 mediated oxidation can effectively introduce carboxyl and carbonyl functional groups into
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fibrous celluloses and activate the cellulose polymers for further chemical modification. It has also been
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investigated that oxidized celluloses may integrate with proteins, dyes, enzymes, or amine drugs by coupling with
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their amino functions to synthesize new type cellulose-based polymers with special performance and application
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(Alinovskaya et al., 1989; Kaputskii et al., 1995; Kumar & Deshpande, 2001).
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In the previous literature (Gert et al., 1995; Kumar & Yang, 2002; Wang et al., 2009; Wei et al., 1996), the
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HNO3/H3PO4–NaNO2 mediated oxidation of natural cellulose is detailedly investigated. However, the study on
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HNO3/H3PO4–NaNO2 oxidation of C6 primary hydroxyls in regenerated cellulose of bamboo pulp fibers has not
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been conducted yet. In our paper, bamboo pulp fibers are used to obtain information about impact of
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HNO3/H3PO4–NaNO2 mediated oxidation on molecular structure and properties of regenerated cellulose fibers. It is
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taken HNO3/H3PO4–NaNO2 as oxidant to oxidize regenerated bamboo pulp fibers under various conditions,
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collected water-insoluble fractions were analyzed in terms of weight loss values, carboxyl contents, fiber fineness
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and mechanical properties. The changes in internal structure of oxidized bamboo pulp fibers were evaluated by
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determination of crystallinity, thermal behavior and surface morphology. The significance of this study is to
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understand the individual roles of introduced functionalities and changed fiber structure in the properties of the
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oxidized bamboo pulp fibers, and achieve a novel biofiber with active carboxyl groups, which can react with other
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functional groups for further surface modification and be utilized in textile and paper industry, medical and packing
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materials.
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2. Experimental
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2.1. Materials
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Bamboo pulp spinning yarns (Lyocell Bamboo Fiber Co., Ltd, Shanghai, China) were used as a regenerated
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cellulose fiber, which was bleached and scoured. Its color was white with a fineness of 37.59 tex. Sodium nitrite,
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68% (w/w) nitric acid solution, 85% (w/w) phosphoric acid solution and acetone were purchased from Aladdin -3-
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Chemical Reagent Co., USA, and other chemicals were obtained from Wako Pure Chemicals, Co., Japan, as
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analytical grades and used without further purification.
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2.2. Preparation of HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers
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The samples of bamboo pulp yarns were soaked in 0.25 M NaOH aqueous solution for 20 min at room
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temperature to remove the oil and impurities, and then washed with deionized water cleanly. Subsequently, a
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sample of bamboo pulp yarns (3.0 g) was immersed in a 90 ml mixture solution of nitric acid and phosphoric acid.
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Once the bamboo pulp fibers were entirely soaked, 0.63 g and 1.26 g (i.e. 0.7% and 1.4%, w/v) of sodium nitrite
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were respectively added immediately. The reaction vessel was covered with a petri dish to prevent reddish brown
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fumes to the air and then the mixture solution was oscillated gently at the absence of light and room temperature, for
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15, 30, 45, 60, 120, 180, 300, 360 and 480 min. After that, the fibers were rinsed thoroughly with deionized water
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several times and soaked in 0.2% (w/w) of propanetriol solution for 15 min to remove the oxidant. It was finally
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washed with acetone and air-dried for about 30 min at 60 ºC and cooled down to room temperature.
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2.3. Measurements and analyses
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2.3.1. Determination of weight loss
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The formation of soluble fragments, as a result of the cellulose chain scission caused by oxidation treatment, was
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determined by measuring the weight loss of oxidized bamboo pulp fibers by applying the direct gravimetric method
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(Koblyakov, 1989).
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2.3.2. Carboxyl (COOH) groups determination
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The amount of carboxyl groups present in the oxidized products was determined according to the reported
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procedure (Yackel & Kenyon, 1942). The carboxyl groups of the oxidized cellulose react with the salts of weaker
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acids such as calcium acetate, forming a salt of the oxidized cellulose and releasing an equivalent amount of the
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weaker acid. For determination of carboxyl content in oxidized bamboo pulp fibers, the following method was
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developed. The fiber samples (0.5 g) were cut up to particles, and then treated with 0.01 M HCl for 1 h for obtaining
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in the acidic form by replacement of its cations by hydrogen ions, followed by washing with deionized water. In the
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next step to the oxidized fibers 50 ml of a 2% (w/w) calcium acetate solution were added. After frequent ultrasonic
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shaking for 30 min, to facilitate completion of the interchange, the mixture was titrated with 0.1 M standardized -4-
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sodium hydroxide, using phenolphthalein as an indicator. The volume of NaOH solution consumed was corrected
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for the blank. The carboxyl content in the sample was calculated as follows:
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Carboxyl content (% w/w) =
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where 0.1 M is the normality concentration of NaOH, VNaOH is the volume (ml) of NaOH solution used in titration
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after correcting for the blank, m is the weight of oxidized bamboo pulp fibers (mg), and w is the moisture content of
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the fibers (%).
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2.3.3. Aldehyde (CHO) groups determination
0.1M × V NaOH × MWCOOH × 100% , m × (1 − w / 100)
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The aldehyde content in oxidized products was determined by the method described by Saito and co-workers
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(2004). The oxidized bamboo pulp fibers were further oxidized with sodium chlorite at pH 4–5 for selective
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conversion of the aldehyde groups in the samples to carboxyl ones, and carboxyl content was determined by above
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mentioned calcium acetate method. Cellulose slurry with 10% consistency was prepared beforehand, and then this
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slurry (20 g) was added to a mixture containing NaClO2 (1.81 g), 5 M CH3COOH (20 g), and water (57 ml).
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Oxidation was performed by stirring the mixture at room temperature for 48 h, followed by washing thoroughly
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with deionized water by filtration. The carboxyl groups formed by the post NaClO2 oxidation of the
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HNO3/H3PO4–NaNO2 oxidized cellulose were regarded as aldehyde groups present in the starting oxidized bamboo
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pulp cellulose.
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2.3.4. Carbonyl groups determination
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The amount of carbonyl groups in oxidized bamboo pulp fibers was measured according to the hydroxylamine
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method (Kumar & Yang, 2002). A 50 ml of hydroxylamine hydrochloride was prepared by dissolving 50 g of
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NH2OH–HCl in 120 ml of 1 M sodium hydroxide solution and then diluting to 1000 ml with water. Subsequently,
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this NH2OH–HCl solution and 0.5 g oxidized fibers were placed in a 250 ml round-bottom flask connected by a
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manifold and with a stopcock for evacuation using a water pump. Then the stopcock was closed and the mixture was
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reacted by heating at 50 ºC for 2 h. When cooling to room temperature, a 25 ml aliquot of the reaction supernatant
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was taken to titrate with a 0.1 M HCl solution to pH 3.2. Using a 25 ml hydroxylamine hydrochloride solution as a
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blank titration was conducted in the same manner. The carbonyl content in the oxidized fibers was calculated using
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the equation.
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0.1M × (V2 − V1 ) × MWCO × 100% , m
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Carbonyl content (% w/w) =
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where 0.1 M is the concentration of HCl, V1 and V2 are the volumes (ml) of 0.1 M HCl consumed in titrations of
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blank and sample solutions, respectively, m is the dry weight of oxidized sample (mg).
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2.3.5. Ketone groups analysis
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The ketone group content in oxidized fibers sample was determined by subtracting the aldehyde group content
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from the carbonyl content confirmed as described above.
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2.3.6. Determination of fineness of oxidized bamboo pulp fibers
Fineness of the oxidized fibers in tex was measured as per standard method (SRPS F.S2.212., 1963) by dividing the mass of fibers by their known length.
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2.3.7. Morphology analysis
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Scanning electron microscopy (SEM) was used to observe longitudinal surface morphology. The samples were
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coated with gold. The instrument was a Japan JEOL JSM-6700F field emission-type scanning electron microscope
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with an accelerating voltage of 5 kV.
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2.3.8. Solid state 13C-CP/MAS NMR analysis
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Solid state 13C-NMR spectra of the oxidized bamboo pulp fibers were recorded using a Bruker AVANCE III-400
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(100.4 MHz) superconducting nuclear magnetic resonance spectrometer (Switzerland) with cross polarization and
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magic angle sample spinning under the following conditions: spinning rate 5 kHz, pulse delay 3 ms, contact time 5
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ms and compensate time 20 ms.
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2.3.9. FT-IR spectroscopy
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The Fourier transform infrared (FT-IR) spectra of samples were obtained on a Nicolet NEXUS-870 FT-IR
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spectrophotometer (Madison, USA) using KBr pellets, and samples were analyzed in the range of wavelength from
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4000 to 400 cm–1 with the result of 24 scans at the resolution of 4 cm–1.
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2.3.10. X-ray diffraction analysis
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X-ray diffraction (XRD) patterns of the bamboo pulp fibers were performed by the reflection method using a -6-
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RINT 2027 X-ray detector diffraction system (Rigaku Corporation, Tokyo, Japan) with a Cu Kα target and β Ni
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filter at a voltage of 40 kV, a current of 30 mA, a scan rate of 2°/min and 2θ range of 5–45°. The crystallinity degree
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(Xc) and crystalline index (CrI) of the fibers sample were calculated by the method of peaks separation and Gaussian
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peak fit with the Peakfit data processing software (Ver. 4.12, Seasolve Software Inc., Framingham, MA, USA)
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according to Eqs. (3) and (4), respectively:
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X c (%) =
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CrI =
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where Sc is the sum of all crystal areas, and Sa is the amorphous area. I002 is the maximum intensity (in arbitrary units)
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of the 002 lattice diffraction and Iam is the diffraction intensity of amorphous peak at about 18° of 2θ.
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2.3.11. Thermal analysis
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Thermogravimetry (TG) and differential thermogravimetry (DTG) analysis were carried out on 4–6 mg samples
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using a SDT Q600 TG/DSC thermal analyzer (TA Instrument Corp., USA) at a heating rate of 10 ºC/min from 20 to
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800 ºC with a constant nitrogen flow of 100 ml/min.
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2.3.12. Mechanical properties
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The tensile breaking strength and elongation of bamboo pulp fibers were taken on a YG021A electronic single
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yarn strength tester (Ningbo Textile Instrument Factory, China). The gauge length of the yarn sample was 200 mm,
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and speed was 200 mm/min. The fiber samples were conditioned in a room at 20 ºC and a relative humidity of 65%
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for 48 h before measurement.
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3. Results and discussion
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3.1. Effect of volume ratios of HNO3 and H3PO4 on the oxidation of bamboo pulp fibers
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For obtaining monocarboxy bamboo pulp cellulose (seen in Scheme 1), the impacts of HNO3/H3PO4–NaNO2
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oxidation on bamboo pulp fibers were assessed by determining the carboxyl group content, weight loss, fiber
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fineness and mechanical properties of fiber samples. In general, since commercial HNO3 contains small amounts of -7-
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nitrogen oxides including NO, NO2, N2O3 N2O5, etc., the oxidation of polysaccharide compounds by HNO3 alone is
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sluggish and frequently requires a catalyst (e.g. H2SO4), an initiator (e.g. NO2 or HNO2), and/or heating at high
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temperature (Ogata, 1978). In this study, the HNO3/H3PO4–NaNO2 mixture system oxidation of regenerated
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cellulose was investigated. The carboxyl content and weight loss of bamboo pulp cellulose oxidized with different
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ratios of HNO3 and H3PO4 at ambient temperature for 6 h are summarized in Table 1. The data presented in Table 1
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display that the ratio of HNO3 and H3PO4 used in the reaction did not have a significant effect on the oxidation of
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bamboo pulp fibers, and the HNO3/H3PO4–NaNO2 oxidation with the ratio of 2:1 (HNO3:H3PO4) obtained the
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maximum carboxyl content of 9.71% and a low weight loss of 17.02%. For comparison purpose, the result of the
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oxidation conducted using the HNO3/H2SO4–NaNO2 reaction system reported by Wanleg (1956) is also presented
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in Table 1. The oxidation with a mixture of HNO3/H2SO4–NaNO2 produced the oxycellulose with 8.75% carboxyl
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content, whereas the weight loss of oxidized bamboo pulp cellulose was found to be very high (about 56.92%). An
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addition in concentration of H2SO4 to speed the oxidation rate would cause further degradation of cellulose and
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hence a further decrease in the yield of oxycellulose (Wanleg, 1956). As shown in Table 1, the reaction system of
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HNO3 in combination with H3PO4 and NaNO2 can achieve the oxidized bamboo pulp fibers with a high carboxyl
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content (7.93–9.71%) and in a relatively low weight loss (15.29–19.03%). These results reveal that the
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HNO3/H3PO4–NaNO2 mixture produced oxycellulose in high yields due to slight hydrolysis of cellulose by a weak
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acid of H3PO4.
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As seen in Table 1, the HNO3/H3PO4–NaNO2 oxidized bamboo pulp celluloses corresponding to different ratios
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of HNO3 and H3PO4 contained 7.93, 8.82, 9.71, 8.48, 8.96, and 8.64% carboxyl contents, respectively. The amounts
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of carbonyl groups determined in these oxidized fibers were 1.83, 2.17, 2.69, 2.08, 2.26, and 2.13%, while the
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carboxyl contents in the samples, after further oxidation with sodium chlorite, corresponded to 7.91, 8.80, 9.71,
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8.47, 8.96, and 8.63%, respectively. These data illustrate that the oxycellulose by HNO3/H3PO4–NaNO2 mediated
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oxidation contained no aldehyde groups and the carbonyl content formed in the oxidized samples was only due to
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ketone groups. Simultaneously the HNO3/H2SO4–NaNO2 mixture oxidized bamboo pulp fibers separately
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contained 8.75% carboxyl content and 3.12% ketone content, and the analysis of carboxyl content in the oxidized
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fibers with sodium chlorite treatment indicated no aldehyde groups. According to the oxidation mechanism for
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alcohols reported by Ogata (1978) the nitrogen oxides have been broadly implicated as the oxidants in the oxidation
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of polysaccharide compounds by HNO3, solely or in the presence of an acid and NaNO2, furthermore, several
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different pathways may cause the generation of these species of nitrogen oxides in situ. During the
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HNO3/H3PO4–NaNO2 mediated oxidation, this reaction is initiated via abstracting a α-hydrogen atom of C6 site
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from cellulose by such odd-electron species of NO2 and NO, giving the product of Cell–C(·)H–OH, further attack
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by NO2 · and subsequent release of NO · and HNO2 forms Cell–CHO and Cell–CH(OH)2 intermediates, respectively.
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Following an attack of NO2 · may lead to a hydrogen abstraction reaction from the Cell–CHO and subsequent
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hydrolysis of the addition product, to yield the Cell–COOH. In the case of Cell–CH(OH)2, it is likely to undergo an
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initial hydrogen abstraction reaction followed by an attack by NO2 · and subsequent elimination of HNO2, resulting
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in the Cell–COOH formation of monocarboxy cellulose.
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3.2. Effects of reaction time and sodium nitrite concentration on the oxidation of bamboo pulp fibers
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The rate of carboxyl group generation in bamboo pulp cellulose by the oxidation from 0.7% and 1.4% sodium
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nitrite in combination with HNO3 and H3PO4 is shown in Fig. 1a. The oxidized bamboo pulp fibers exhibited
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increase in carboxyl content ranging from 0.95% to 10.38% with the increase of oxidation time and concentration of
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NaNO2. During the initial 60 min, the values of carboxyl group content in fibers oxidized by HNO3/H3PO4–NaNO2
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mixture using different concentrations of NaNO2 are very similar, and the reaction rate is fast. Subsequently
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increasing the reaction time, from 60 to 360 min, the HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers had higher
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carboxyl content compared to the fibers oxidized in the first stage, and the bamboo pulp fibers oxidized with 1.4%
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NaNO2 presented higher carboxyl content than the fibers oxidized with 0.7% NaNO2. Finally extending the
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oxidation time up to 480 min, caused only a small increase in the carboxyl content. The oxidation rate gradually
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decreased and the carboxyl content of bamboo pulp fibers oxidized with 0.7% and 1.4% NaNO2 became closer.
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During the HNO3/H3PO4–NaNO2 reaction, bamboo pulp cellulose chain-molecules would undergo scission with
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the production of soluble fragments due to side reactions of oxidation. The weight loss of oxidized bamboo pulp
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fibers may reflect not only the formation of soluble fragments but also the oxidative extent of the fibers oxidized by
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HNO3/H3PO4–NaNO2, as it is shown in Fig. 1b. The loss in weight of oxidized bamboo pulp fibers increased with
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the extended oxidation time, especially pronounced when the time of oxidation was over 60 min. Furthermore, the
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weight loss with values up to ~25.26% of the fibers oxidized by higher sodium nitrite charge (1.4% NaNO2)
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combining with HNO3 and H3PO4 is more noticeable than that (ranging in 3.48–15.72%) of the cellulose fibers
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oxidized with low sodium nitrite charge. In addition, the bamboo pulp fibers contraction phenomenon can be
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observed in the oxidation process of HNO3/H3PO4–NaNO2 mixture. Fig. 1c clarifies that the fineness of oxidized
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bamboo pulp fibers basically increased with higher NaNO2 concentration and increasing reaction time, and the -9-
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value of the oxidized fibers fineness was enhanced up to 13.81%, namely from 37.59 tex for starting (non-oxidized)
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fibers to 42.78 tex for the fibers oxidized by 1.4% NaNO2 during 480 min. Theoretically, increasing oxidation time may increase the carboxyl content of the oxidized bamboo pulp fibers
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during the HNO3/H3PO4–NaNO2 mediated oxidation process, and high carboxyl content may subsequently enhance
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the ability of further chemical modification for the oxidized fibers. However, the oxidation can break to some extent
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the crystal structure in the original cellulose (Saito & Isogai, 2004). Figs. 1d and e show the effect of the oxidation
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time on the mechanical properties of the oxidized fibers. In contrast with the starting bamboo pulp fibers, the
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breaking strength and elongation of oxidized fibers decreased with prolonging time. The tensile strength and
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elongation of the oxidized fibers did not change remarkably for the oxidation time in the range of 0–60 min,
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whereas it dramatically decreased when the oxidation time was over 60 min. The bamboo pulp fibers oxidized by
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1.4% NaNO2 had much lower tensile strength and elongation than the fibers oxidized by 0.7% NaNO2 during the
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severe oxidation degree, and the fibers oxidized with higher sodium nitrite concentration (1.4% NaNO2) and longer
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oxidation time (480 min) became so rigid and crisp that their tensile strength and elongation could not be surveyed,
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consequently, no data are shown in Figs. 1d and e. From these results it is clear that slight oxidation of
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HNO3/H3PO4–NaNO2 for 60 min with a ratio of 2:1 (v/v) of nitric acid and phosphoric acid containing 1.4% (w/v)
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of sodium nitrite yields adequate carboxyl content in the oxidized cellulose to ensure larger reaction activity for the
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oxidized fibers when applied in more potential chemical modification. Meanwhile, the tensile properties of the
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bamboo pulp fibers with slight oxidation had no significant change, which are suitable for dry goods and medical
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textile applications.
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3.3. Morphology analysis of oxidized bamboo pulp fibers
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These micrographs obtained accordingly from the starting bamboo pulp fiber, the oxidized fibers with different
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oxidation time are shown in Fig. 2. It can be seen from Fig. 2a that the non-oxidized bamboo pulp fiber consists of
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fibrils which is basically arranged along the length of the fiber. There are full of grooves between fibrils in the
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longitudinal fiber surface and apertures along the length of the fibrils, and its morphology is straight stripes with a
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sleek shape. Figs. 2b and c show that bamboo pulp fibers were suffered from mild corrosion of oxidant in the
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oxidation process of short time, some grooves on longitudinal surface of the oxidized fiber became deeper and
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wider, but the whole surface morphology is almost similar to the original fiber.
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Further extending the oxidation time, the HNO3/H3PO4–NaNO2 system oxidized fibers have a great different - 10 -
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longitudinal morphology as shown in Figs. 2(d, e and f) in comparison with non-oxidized ones. In Fig. 2d, the
270
oxidized fiber surface is very rough, and many narrow lines and apertures affected by oxidation can be easily seen,
271
some fibers start to generate slight convolution. The longitudinal surface of the bamboo pulp fiber with high
272
oxidation degree of long time is tightly contracted and crimpled to some extent as shown in Fig. 2e. So it could be
273
seen the grooves and fibril stripe outlines by spells. In the same way, Fig. 2f also represents the surface
274
characteristics like in Fig. 2e, but there is an addition in large extent, and some small crevices and holes appear on
275
the partial surface of the oxidized fiber. So there is nearly no sign of grooves between fibrils and the smooth and
276
sleek straight stripes become illegible. This indicates that the acute oxidation of long time by HNO3/H3PO4–NaNO2
277
mixture may result in the increase of weakness on the fiber surface, that are responsible for a decrease in tensile
278
properties of the oxidized bamboo pulp fibers.
279
3.4. FT-IR characterization
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Fig. 3 shows FT-IR spectra of non-oxidized bamboo pulp fiber and oxidized fibers with different oxidation time.
281
All of them had a broad peak corresponding to –OH group stretching of cellulose observed in the range 3100–3500
282
cm–1. However, these absorption peaks of –OH stretching in oxidized fibers gradually became narrower and
283
transferred to higher wavenumber position when increased the oxidation time. One possible reason for this could be
284
that the hydrogen bonding between cellulose chains was weakened, loosening the crystalline structure of fibers due
285
to deep oxidation for long time. For the bamboo pulp fibers oxidized for different oxidation time, a sharp band
286
associated with –OH group in-plane bending of cellulose at 1377.1 cm–1, a peak at around 1161.2 cm–1 due to
287
C–O–C asymmetric stretching and a little peak at 1030.2 cm–1 due to C–O stretching totally decreased somewhat in
288
the intensity. But another sharp band corresponding to –CH group bending of cellulose at about 1284.5 cm–1 was
289
very stronger than that of original bamboo pulp fiber. Compared with non-oxidized bamboo pulp fiber, the infrared
290
spectra of oxidized samples in Figs. 3(b–e) exhibit that a sharp characteristic absorption peak corresponding to C=O
291
stretching vibration of carbonyl group clearly appeared at 1739.7 cm–1 or so, whose absorptivity enhanced with
292
extending oxidation time, consistent with the increase of the carboxyl and ketone groups in oxidized fibers. This
293
should be attributed to the formation of carboxyl and carbonyl groups in the bamboo pulp fibers with
294
HNO3/H3PO4–NaNO2 mediated oxidation.
295
3.5. X-ray diffraction analysis
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Fig. 4 shows the X-ray diffraction spectra of bamboo pulp fiber samples treated with different oxidation time.
297
The diffraction patterns of oxidized fibers are similitude with that of the non-oxidized bamboo pulp fiber. It is well
298
known that the cellulose I structure of the native cellulose is changed to the cellulose II structure by cellulose
299
regeneration. Bamboo pulp fiber was regenerated one, so it was the cellulose II structure (Yang et al., 2008). The
300
locations of the characteristic peaks of bamboo pulp cellulose were obtained by deconvoluting the XRD spectra
301
with the Peakfit software. The diffractive peaks of oxidized bamboo pulp fibers at 2θ values of 12.4º and 20.2º were
302
respectively attributed to the (110) and (002) planes of the typical cellulose II structure. Hence it can be deduced
303
that the crystalline forms of the bamboo pulp fibers did not markedly change in the processing of
304
HNO3/H3PO4–NaNO2 mediated oxidation.
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Additionally, the crystallinity and crystalline index were calculated by foregoing Eqs. (3) and (4) after the
306
wide-angle X-ray diffraction (WAXD) patterns were deconvoluted by using the Gaussian deconvolution method
307
and the diffraction peaks numerical fitting ratio achieved 99.99% through the PeakFit software, and the results were
308
listed in Table 2. The data exhibited that the crystallinity and crystalline index of bamboo pulp fibers progressively
309
decreased with increasing oxidation time. This reason may be that the oxidation reaction of HNO3/H3PO4–NaNO2
310
mixture destroyed hydrogen bonding between bamboo pulp cellulose chains in both crystal and amorphous regions
311
during oxidizing in the range of our experiment conditions, which affects its physical properties, and that is
312
consistent with the above results from mechanical properties analysis of oxidized bamboo pulp fibers.
313
3.6. Solid-state 13C-NMR analysis of HNO3/H3PO4–NaNO2 oxidized cellulose
Ac ce p
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314
In HNO3/H3PO4–NaNO2 mediated oxidation of cellulose, the resonance signals of carbons in pyranose units of
315
bamboo pulp cellulose can be monitored by the solid-state CP/MAS 13C-NMR method. It can be comprehended
316
from Fig. 5 that the original and HNO3/H3PO4–NaNO2 oxidized bamboo pulp celluloses had broad C1,
317
non-crystalline C4, crystalline C6 and C2,3,5 signals, while small signal due to crystalline C4 and non-crystalline
318
C6 of cellulose II remained as well. The apparent C1 signal of the original bamboo pulp fiber at 102–108 ppm was
319
basically unchanged. The overlapped signals due to C2, C3 and C5 appeared at 69–81 ppm. The chemical shift and
320
intensities of C2, C3 and C5 signals somewhat changed by the oxidation. The signals for crystalline and
321
non-crystalline C4 carbons were located at 87–92 and 81–87 ppm, respectively (Isogai et al., 1989). The shape and
322
ratio of crystalline and non-crystalline C4 signals of the original bamboo pulp cellulose were roughly unchanged
323
even after the HNO3/H3PO4–NaNO2 oxidation for 6 h. These results show that the HNO3/H3PO4–NaNO2 mediated - 12 -
Page 12 of 30
324
oxidation of bamboo pulp fibers has nearly no influences on the chemical shift and the patterns of C1 and C4 in
325
solid-state
326
60–69 ppm, and the signal tops for crystalline and non-crystalline C6 carbons were respectively located at 64 and 61
327
ppm (Kumar & Kothari, 1999), the crystalline C6 signal gradually decreased with increasing the
328
HNO3/H3PO4–NaNO2 oxidation time, while the shoulder signal due to non-crystalline C6 carbons at 61 ppm
329
enhanced to some extent. Meanwhile, the signals due to carboxyl carbons in the oxidized fibers appeared at around
330
175 ppm, and their intensities increased with extended oxidation time. Probably the carboxyl groups formed in the
331
HNO3/H3PO4–NaNO2 oxidized bamboo pulp celluloses are mostly present at the C6 position, and the
332
HNO3/H3PO4–NaNO2 mediated oxidation at the C6 position primary hydroxyls are progressed in the
333
non-crystalline region and crystal zone of cellulose II of bamboo pulp fibers, in coincidence with those from XRD
334
analysis.
335
3.7. Thermal analysis
C-NMR spectra. On the other hand, the C6 resonance signal in bamboo pulp cellulose appeared at
M
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13
Fig. 6 shows the TG (a) and DTG (b) curves of the original bamboo pulp fiber and oxidized bamboo pulp fibers,
337
and the temperatures at which the major weight losses are also exhibited in Table 3. It was observed that all bamboo
338
pulp fiber samples showed a weight loss from 45 °C to 125 ºC due to the moisture content vaporization, and the
339
moisture content in these celluloses increased with adding oxidation extent (seen in Table 3). This is because the
340
bamboo pulp fibers with higher carboxyl content had the stronger affinity for water molecules. In the initial stage of
341
weight loss (180–380°C), almost all oxidized fiber samples started to decompose at lower temperatures than the
342
starting bamboo pulp cellulose, and the fastest thermo-decomposition temperature for the original fiber was 340 °C,
343
while they decreased to 235–318 °C for the oxidized products, again reflecting their lower crystallinity compared
344
with the original cellulose. Furthermore, the decomposition patterns became more complex with extending
345
oxidation degree, and two-stage decompositions occurred for all oxidized bamboo pulp materials during initial
346
weight losses (up to about 380 °C). The final stage was that the crystal region of the fibers had been completely
347
destructed and the cellulose decomposed totally from 380 ºC to the termination of 800 ºC. The final weight loss of
348
all oxidized fiber samples was in the 72–79% range, while for the starting bamboo pulp fiber the value was 86%.
349
Although the TG curves of the original bamboo pulp fiber and the oxidized fiber series are similar in some way, the
350
temperatures of the oxidized bamboo pulp fibers at the beginning degradation and maximal thermo-decomposition
351
rate are lower than those of the starting bamboo pulp fiber, so the thermal stability of these oxidized bamboo pulp
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352
fibers decreased with enhancing oxidation extent.
353
4. Conclusions The results presented in this study show that the HNO3/H3PO4–NaNO2 mediated oxidation of bamboo pulp fibers
355
with different sodium nitrite concentrations and reaction time can obtain oxidized fiber products with high yield and
356
various carboxyl content. It is manifested that the different ratios of HNO3 and H3PO4 used in the oxidation reaction
357
did not significantly influence the weight loss and carboxyl content of oxidized bamboo pulp fibers. Since the
358
carboxyl content and weight loss of the oxidized fiber increased with adding oxidation time, we can easily acquire
359
the oxidized bamboo pulp fiber with expectant yield and carboxyl content by controlling the oxidation time of
360
HNO3/H3PO4–NaNO2 mixture. With increasing reaction time, the fiber fineness increased and mechanical
361
properties of the oxidized bamboo pulp fiber reduced on the whole.
an
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354
From the analysis of structure, it can be deduced that the crystallinity of the oxidized bamboo pulp cellulose
363
decreased gradually with increasing oxidation extent, but the typical cellulose II structure was not changed. The
364
results from TG and DTG show that the oxidized fiber materials had a lower thermal stability than the original
365
bamboo pulp fiber. Solid-state 13C-NMR and X-ray diffraction analyses revealed that the carboxyl and carbonyl
366
groups introduced into the HNO3/H3PO4–NaNO2 oxidized celluloses were mostly present at the C6 position of
367
amorphous and crystalline regions in bamboo pulp fibers. In addition, the tensile properties of oxidized fibers were
368
not changed very much, and the oxidized fiber materials containing 1.13–3.56% of carboxyl content and with
369
94–98% of high yields can be obtained in the range of oxidation time from 15 to 60 min. It can be implied that the
370
resultant oxidized bamboo pulp fibers possess more potential for still more chemical modification due to their
371
greater active carboxyl and carbonyl groups and may find extensive applications in textile and medical industries.
372
Acknowledgements
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This research has been supported by Scientific Research Foundation for Returned Scholars of Ministry of
374
Education of China (Grant number (2011)1568), Anhui Provincial Natural Science Foundation of China (Grant
375
number 10040606Q16), and Anhui Provincial Higher Academy Natural Science Research Key Program of China
376
(Grant number KJ2012A116).
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Page 14 of 30
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Kumar, V., & Yang, T. (2002). HNO3/H3PO4–NaNO2 mediated oxidation of cellulose-preparation and
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nitrogen dioxide/carbon tetrachloride. Fibers and Polymers, 13(5), 576–581. Yackel, E. C., & Kenyon, W. O. (1942). The oxidation of cellulose by nitrogen dioxide. Journal of the American
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Figures
445
Scheme 1. HNO3/H3PO4–NaNO2 mediated oxidation of bamboo pulp cellulose.
446 447 448
Fig. 1. Relationship between the carboxyl group content (a), weight loss (b), fiber fineness (c), breaking strength (d) and breaking elongation (e) of the oxidized bamboo pulp fibers and the oxidation time and the concentration of NaNO2 used for the oxidation.
449 450
Fig. 2. SEM micrographs of: (a) non-oxidized bamboo pulp fiber, (b) fiber oxidized for 15 min, (c) fiber oxidized for 60 min, (d) fiber oxidized for 180 min, (e) fiber oxidized for 300 min, and (f) fiber oxidized for 360 min.
451 452
Fig. 3. FT-IR spectra of: (a) starting bamboo pulp fiber, (b) fiber oxidized for 60 min, (c) fiber oxidized for 180 min, (d) fiber oxidized for 300 min, and (e) fiber oxidized for 360 min.
453 454
Fig. 4. X-ray diffraction patterns of bamboo pulp fibers oxidized with different oxidation time: (a) 0 min, (b) 60 min, (c) 180 min, (d) 300 min, and (e) 360 min.
455 456
Fig. 5. Solid-state CP/MAS 13C-NMR spectra of the original bamboo pulp fiber and HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers under various conditions.
457
Fig. 6. TG (a) and DTG (b) curves of the starting bamboo pulp fiber and a series of oxidized bamboo pulp fibers.
us
cr
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444
an
458 459 460 461
M
462 463
467 468 469 470 471 472 473 474 475 476
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466
Ac ce p
465
d
464
477 478 479 480 481 482 483 484 485 - 18 -
Page 18 of 30
cr
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d
M
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Scheme 1. HNO3/H3PO4–NaNO2 mediated oxidation of bamboo pulp cellulose.
Ac ce p
493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531
ip t
486 487 488 489 490 491 492
- 19 -
Page 19 of 30
532 533
a 12 10
1.4% NaNO2
6 4
20 15
ip t
8
10
cr
5
2
0
0
0.7% NaNO2 1.4% NaNO2
420 410
M
400
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
390 380
d
370 360
te
Fineness of oxidized fibers (dtex)
c 430
0
us
534 535
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
an
0
0
d 1.6
e 16
0.7% NaNO2
0.7% NaNO2
14
1.4% NaNO2
1.4
Tensile elongation (%)
Tensile strength (cN/dtex)
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
Ac ce p
536 537
1.2 1.0 0.8 0.6 0.4
1.4% NaNO2
12 10 8 6 4 2
0.2
0
0.0 0
538 539 540 541 542 543 544 545 546 547
0.7% NaNO2
1.4% NaNO2
Weight loss (%)
Carboxyl content (%)
b 25
0.7% NaNO2
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
0
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
Fig. 1. Relationship between the carboxyl group content (a), weight loss (b), fiber fineness (c), breaking strength (d) and breaking elongation (e) of the oxidized bamboo pulp fibers and the oxidation time and the concentration of NaNO2 used for the oxidation.
- 20 -
Page 20 of 30
a
b
c
d
e
557 558 559 560 561 562 563 564 565 566 567 568 569 570 571
us
f
Ac ce p
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555 556
d
M
an
553 554
cr
ip t
548 549 550 551 552
Fig. 2. SEM micrographs of: (a) non-oxidized bamboo pulp fiber, (b) fiber oxidized for 15 min, (c) fiber oxidized for 60 min, (d) fiber oxidized for 180 min, (e) fiber oxidized for 300 min, and (f) fiber oxidized for 360 min.
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Page 21 of 30
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Fig. 3. FT-IR spectra of: (a) starting bamboo pulp fiber, (b) fiber oxidized for 60 min, (c) fiber oxidized for 180 min, (d) fiber oxidized for 300 min, and (e) fiber oxidized for 360 min.
Ac ce p
578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610
an
us
cr
ip t
572 573 574 575 576 577
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Page 22 of 30
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Fig. 4. X-ray diffraction patterns of bamboo pulp fibers oxidized with different oxidation time: (a) 0 min, (b) 60 min, (c) 180 min, (d) 300 min, and (e) 360 min.
Ac ce p
617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649
an
us
cr
ip t
611 612 613 614 615 616
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Page 23 of 30
te
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Fig. 5. Solid-state CP/MAS 13C-NMR spectra of the original bamboo pulp fiber and HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers under various conditions.
Ac ce p
656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688
an
us
cr
ip t
650 651 652 653 654 655
- 24 -
Page 24 of 30
689 690 691 692 693 694
a
an
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Fig. 6. TG (a) and DTG (b) curves of the starting bamboo pulp fiber and a series of oxidized bamboo pulp fibers.
Ac ce p
695 696 697 698 699
us
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b
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Page 25 of 30
Tables
700 701
Table 1 Effect of different ratios of HNO3 and H3PO4 (or H2SO4) on oxidation of bamboo pulp cellulose
702 703 704
Table 2 Characteristic peaks and crystalline parameters of bamboo pulp fibers oxidized with different oxidation time obtained from WAXD.
705 706
Table 3 Thermogravimetric data on the starting bamboo pulp fiber and oxidized bamboo pulp fiber samples
cr us an M d te Ac ce p
707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754
ip t
699
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Page 26 of 30
Table 1 Effect of different ratios of HNO3 and H3PO4 (or H2SO4) on oxidation of bamboo pulp cellulose Carboxyl contentb (%)
Carbonyl contentc (%)
Oxidation system: HNO3/H3PO4–NaNO2d A 11:1 B 4:1 C 2:1 D 1:1 E 1:2 F 1:4
16.34 15.29 17.02 17.80 19.03 18.68
7.93 8.82 9.71 8.48 8.96 8.64
1.83 2.17 2.69 2.08 2.26 2.13
Oxidation system: HNO3/H2SO4–NaNO2 G 11:1e
56.92
8.75
a
3.12
us
Represents average value of 3 determinations, standard deviation (%) range ±0.68–1.19. Represents average value of 3 determinations, standard deviation (%) range ±0.14–0.47. c Represents average value of 3 determinations, standard deviation (%) range ±0.11–0.91. d All oxidations were proceeded at room temperature for 6 h with a bath ratio of 1:30 (w/v) of bamboo pulp fiber to acid solution and about 1.4% (w/v) of NaNO2. e Corresponds to the same ratio used by Wanleg (1956). The ratio of bamboo pulp fiber, acid mixture and NaNO2 was as same as the HNO3/H3PO4–NaNO2 reaction system.
te
d
M
an
b
Ac ce p
760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804
HNO3:H3PO4 (or H2SO4) (v/v)
ip t
Weight lossa (%)
Oxidation No.
cr
755 756 757 758 759
- 27 -
Page 27 of 30
Location of characteristic peak, 2θ (º)
a b c d e
12.62 12.43 12.36 12.30 12.51
16.56 16.47 16.29 16.72 16.34
20.28 20.25 20.34 20.12 20.14
22.02 22.16 22.10 22.34 22.27
Crystalline index (%)
44.98 40.43 38.21 30.69 29.14
52.24 49.68 45.97 32.41 28.03
d
M
an
us
cr
15.40 14.82 14.64 14.94 15.48
Crystallinity (%)
ip t
Sample No.
te
811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858
Table 2 Characteristic peaks and crystalline parameters of bamboo pulp fibers oxidized with different oxidation time obtained from WAXD.
Ac ce p
805 806 807 808 809 810
- 28 -
Page 28 of 30
Table 3 Thermogravimetric data on the starting bamboo pulp fiber and oxidized bamboo pulp fiber samples
Original bamboo pulp fiber 1 h-oxidation 3 h-oxidation 5 h-oxidation 6 h-oxidation
Final weight loss
Degradation temperaturea (°C)
Weight loss (%)
Degradation temperaturea (°C)
Weight loss (%)
10.78
278.8
63.67
369.5
85.62
13.22 14.58 16.76 17.31
236.7, 286.6 216.3, 263.2 194.4, 279.1 190.2, 266.3
55.40 56.49 60.57 59.21
363.3 371.2 386.7 378.9
75.54 72.01 79.08 78.27
d
M
an
us
Two or more temperatures indicate two or more stages of decomposition.
te
a
Initial weight loss
Ac ce p
864 865 866
Volatile content (%)
ip t
Sample No.
cr
859 860 861 862 863
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Highlights H3PO4/HNO3–NaNO2 mediated oxidation of bamboo pulp cellulose obtains 6-carboxycellulose. Supermolecular structure of the oxidized cellulose was characterized. Correlations on structure and properties of bamboo pulp cellulose fibers during oxidation were investigated. Oxidation mostly occurred at C6 primary hydroxyls of non-crystalline and crystal regions of cellulose.
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S0144-8617(14)00499-8 http://dx.doi.org/doi:10.1016/j.carbpol.2014.05.029 CARP 8896
To appear in: Received date: Revised date: Accepted date:
4-2-2014 26-4-2014 3-5-2014
Please cite this article as: Xu, Y., Liu, X., Liu, X., Tan, J., & Zhu, H.,Influence of HNO3 /H3 PO4 ndashNANO2 mediated oxidation on the structure and properties of cellulose fibers, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.05.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Influence of HNO3/H3PO4–NANO2 mediated oxidation on the structure and properties of cellulose fibers
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Yunhui Xu a,*, Xin Liu b, Xuelan Liu c, Jiulong Tan a, Hongling Zhu a
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a
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The bamboo pulp cellulose fiber was oxidized with HNO3/H3PO4–NaNO2 mixture to obtain oxidized cellulose
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containing different levels of carboxyl content and with high yields. The effects of HNO3/H3PO4–NaNO2 mediated
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oxidation on structure and properties of the fiber were investigated. The results showed that an increase in carboxyl
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content and weight loss of oxidized fibers appeared with increasing oxidation time. Compared with the original
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cellulose, the oxidized fibers had lower crystallinity (29–40%) and thermal stability. The patterns of
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X-ray diffraction and other testing methods revealed that the oxidation mostly occurred at C6 primary hydroxyl
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groups of cellulose. Moreover, an oxidized fiber with 94.14–98.59% of high yields and 1.13–3.56% of carboxyl
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content was obtained in the range of oxidation time from 15 to 60 min, while its mechanical properties did not
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change significantly. This work presented some detailed information about structure-property correlations of
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oxidized bamboo pulp cellulose fibers and was useful in planning applications of these products.
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Keywords: Bamboo pulp cellulose; Oxidation of C6 position; Monocarboxy cellulose; Crystalline structure; NMR
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1. Introduction
College of Light-Textile Engineering and Art, Anhui Agricultural University, Hefei, Anhui 230036, China College of Natural Sciences, Anhui Agricultural University, Hefei, Anhui 230036, China c Key Laboratory of Zoonoses of Anhui Province, Anhui Agricultural University, Hefei, Anhui 230036, China
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C-NMR,
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Abstract
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The unrenewable resources of petroleum and coal become increasingly exhausted, so there is a growing attention
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to utilize natural biopolymers. In this regard, fiber plants consisting of cellulose, hemicelluloses and lignin acquire
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enormous significance as rich biomass and chemical feedstock since they are containing many functional groups
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suitable to chemical derivatization (Khullar et al., 2008). Bamboo is a woody herbaceous plant and consists of many
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vascular bundles and xylem. Cellulose pulp prepared from bamboo is around 57–66%, and especially α-cellulose
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gets about 26–43% that is comparable with that of softwoods and hardwoods (He et al., 2008; Shanmughave, 2003).
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Compared with other counterpart timber species, bamboo is an abundant renewable natural resource capable of fast
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growth (3–5 years), high productivity and holocellulose content (64–70%), and easy propagation, which is not like * Corresponding author. Tel.: +86 551 6578 6459; fax: +86 551 6578 6331. E-mail address: [email protected] (Y.H. Xu).
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wood to suffer insect infestation. Bamboo pulp cellulose fiber is a novel kind of green and environment-friendly
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fiber that has been successfully industrialized in China. At present, the regenerated bamboo cellulose fiber is
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produced in the form of dissolving pulp of bamboo cellulose by using viscose or Lyocell spinning procedure.
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However, it is very different from traditional viscose fiber in the raw material and process. Bamboo pulp fiber has
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markedly different properties from cotton and wood fibers, such as lustrous looking, soft and cool feelings, good
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drapability, high water and perspiration adsorption, abrasion resistance, good dyeability, anti-ultraviolet, and
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excellent thermal conductivity (Ma et al., 2011; Shen et al., 2004). Bamboo pulp cellulose fibers have already
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attained an increasingly important position in the clothing and medical textile industry.
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Chemical modification of polysaccharides is an important approach for preparation of new materials with
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improved performance and value-addition (Liu & Sun, 2008; Schurz, 1999). In principle, chemical modification of
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native and regenerated cellulose fibers by selective oxidation reactions of primary and secondary hydroxyl groups
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of the pyranose ring in cellulose offers an attractive means for acquisition of new properties by introduction of
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aldehyde and carboxyl active groups to a polymer surface (Gert et al., 2005; Montanari et al., 2005). The partial
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oxidation renders cellulose bioabsorbable in human, on such way introduced functionalities further broadens the
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potential applications of these biodegradable polymers, and it has been applied in medical devices such as
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absorbable adhesion barriers and hemostatic agents, scaffold in tissue engineering, drug delivery matrix, biosensors,
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etc., while the hemostatic properties and absorption of oxycellulose can be adjusted by the degree of oxidation (Hon,
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1996).
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Oxidized cellulose containing carboxyl groups is commercially available by reacting cellulose with NO2 or N2O4
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in the gaseous form or as a solution in an appropriate organic solvent (Ashton, 1968; Bertocchi et al., 1995; Wu et
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al., 2012). Other oxidants usually applied to produce monocarboxy cellulose include HNO3, HNO3–NaNO2,
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HNO3/H2SO4–NaNO2, H3PO4–NaNO2 and H3PO4–NaNO2–NaNO3, etc (Bertocchi et al., 1995; Gert et al., 1995;
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Painter et al., 1985; Wanleg, 1956). The reaction with HNO3 alone or a mixture of H3PO4–NaNO2 requires heating
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at high temperature, these cause extensive hydrolysis of cellulose and low yield of oxycellulose. The HNO3–NaNO2
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can prepare oxycellulose with 14–18% carboxyl content, but requires a long oxidation time. The mediated
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oxidation of HNO3/H2SO4–NaNO2 is quicker compared to that with HNO3–NaNO2 alone and achieves the oxidized
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product with up to ~21% carboxyl content. However, the yield of oxycellulose was decreased to about 20–30% due
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to further hydrolysis by strong acid of H2SO4. Oxidation of primary hydroxyl groups of celluloses using a mixture
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of HNO3/H3PO4–NaNO2 as an oxidant has become one of the interesting routes to introduce carbonyl and carboxyl -2-
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functionalities into polysaccharides (Kumar & Yang, 2002; Painter et al., 1985; Wang et al., 2009). The advantages
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of HNO3/H3PO4–NaNO2 mediated oxidation of polysaccharides are the followings: high selectivity and yield, just
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modest degradation of celluloses through out the oxidation process, and this oxidation proceeds under aqueous mild
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conditions around room temperature (Kumar & Yang, 2002; Saito et al., 2006). On the other hand, the
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HNO3/H3PO4–NaNO2 mediated oxidation can effectively introduce carboxyl and carbonyl functional groups into
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fibrous celluloses and activate the cellulose polymers for further chemical modification. It has also been
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investigated that oxidized celluloses may integrate with proteins, dyes, enzymes, or amine drugs by coupling with
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their amino functions to synthesize new type cellulose-based polymers with special performance and application
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(Alinovskaya et al., 1989; Kaputskii et al., 1995; Kumar & Deshpande, 2001).
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In the previous literature (Gert et al., 1995; Kumar & Yang, 2002; Wang et al., 2009; Wei et al., 1996), the
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HNO3/H3PO4–NaNO2 mediated oxidation of natural cellulose is detailedly investigated. However, the study on
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HNO3/H3PO4–NaNO2 oxidation of C6 primary hydroxyls in regenerated cellulose of bamboo pulp fibers has not
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been conducted yet. In our paper, bamboo pulp fibers are used to obtain information about impact of
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HNO3/H3PO4–NaNO2 mediated oxidation on molecular structure and properties of regenerated cellulose fibers. It is
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taken HNO3/H3PO4–NaNO2 as oxidant to oxidize regenerated bamboo pulp fibers under various conditions,
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collected water-insoluble fractions were analyzed in terms of weight loss values, carboxyl contents, fiber fineness
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and mechanical properties. The changes in internal structure of oxidized bamboo pulp fibers were evaluated by
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determination of crystallinity, thermal behavior and surface morphology. The significance of this study is to
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understand the individual roles of introduced functionalities and changed fiber structure in the properties of the
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oxidized bamboo pulp fibers, and achieve a novel biofiber with active carboxyl groups, which can react with other
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functional groups for further surface modification and be utilized in textile and paper industry, medical and packing
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materials.
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2. Experimental
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2.1. Materials
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Bamboo pulp spinning yarns (Lyocell Bamboo Fiber Co., Ltd, Shanghai, China) were used as a regenerated
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cellulose fiber, which was bleached and scoured. Its color was white with a fineness of 37.59 tex. Sodium nitrite,
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68% (w/w) nitric acid solution, 85% (w/w) phosphoric acid solution and acetone were purchased from Aladdin -3-
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Chemical Reagent Co., USA, and other chemicals were obtained from Wako Pure Chemicals, Co., Japan, as
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analytical grades and used without further purification.
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2.2. Preparation of HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers
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The samples of bamboo pulp yarns were soaked in 0.25 M NaOH aqueous solution for 20 min at room
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temperature to remove the oil and impurities, and then washed with deionized water cleanly. Subsequently, a
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sample of bamboo pulp yarns (3.0 g) was immersed in a 90 ml mixture solution of nitric acid and phosphoric acid.
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Once the bamboo pulp fibers were entirely soaked, 0.63 g and 1.26 g (i.e. 0.7% and 1.4%, w/v) of sodium nitrite
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were respectively added immediately. The reaction vessel was covered with a petri dish to prevent reddish brown
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fumes to the air and then the mixture solution was oscillated gently at the absence of light and room temperature, for
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15, 30, 45, 60, 120, 180, 300, 360 and 480 min. After that, the fibers were rinsed thoroughly with deionized water
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several times and soaked in 0.2% (w/w) of propanetriol solution for 15 min to remove the oxidant. It was finally
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washed with acetone and air-dried for about 30 min at 60 ºC and cooled down to room temperature.
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2.3. Measurements and analyses
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2.3.1. Determination of weight loss
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The formation of soluble fragments, as a result of the cellulose chain scission caused by oxidation treatment, was
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determined by measuring the weight loss of oxidized bamboo pulp fibers by applying the direct gravimetric method
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(Koblyakov, 1989).
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2.3.2. Carboxyl (COOH) groups determination
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The amount of carboxyl groups present in the oxidized products was determined according to the reported
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procedure (Yackel & Kenyon, 1942). The carboxyl groups of the oxidized cellulose react with the salts of weaker
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acids such as calcium acetate, forming a salt of the oxidized cellulose and releasing an equivalent amount of the
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weaker acid. For determination of carboxyl content in oxidized bamboo pulp fibers, the following method was
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developed. The fiber samples (0.5 g) were cut up to particles, and then treated with 0.01 M HCl for 1 h for obtaining
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in the acidic form by replacement of its cations by hydrogen ions, followed by washing with deionized water. In the
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next step to the oxidized fibers 50 ml of a 2% (w/w) calcium acetate solution were added. After frequent ultrasonic
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shaking for 30 min, to facilitate completion of the interchange, the mixture was titrated with 0.1 M standardized -4-
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sodium hydroxide, using phenolphthalein as an indicator. The volume of NaOH solution consumed was corrected
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for the blank. The carboxyl content in the sample was calculated as follows:
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Carboxyl content (% w/w) =
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where 0.1 M is the normality concentration of NaOH, VNaOH is the volume (ml) of NaOH solution used in titration
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after correcting for the blank, m is the weight of oxidized bamboo pulp fibers (mg), and w is the moisture content of
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the fibers (%).
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2.3.3. Aldehyde (CHO) groups determination
0.1M × V NaOH × MWCOOH × 100% , m × (1 − w / 100)
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The aldehyde content in oxidized products was determined by the method described by Saito and co-workers
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(2004). The oxidized bamboo pulp fibers were further oxidized with sodium chlorite at pH 4–5 for selective
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conversion of the aldehyde groups in the samples to carboxyl ones, and carboxyl content was determined by above
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mentioned calcium acetate method. Cellulose slurry with 10% consistency was prepared beforehand, and then this
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slurry (20 g) was added to a mixture containing NaClO2 (1.81 g), 5 M CH3COOH (20 g), and water (57 ml).
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Oxidation was performed by stirring the mixture at room temperature for 48 h, followed by washing thoroughly
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with deionized water by filtration. The carboxyl groups formed by the post NaClO2 oxidation of the
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HNO3/H3PO4–NaNO2 oxidized cellulose were regarded as aldehyde groups present in the starting oxidized bamboo
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pulp cellulose.
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2.3.4. Carbonyl groups determination
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The amount of carbonyl groups in oxidized bamboo pulp fibers was measured according to the hydroxylamine
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method (Kumar & Yang, 2002). A 50 ml of hydroxylamine hydrochloride was prepared by dissolving 50 g of
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NH2OH–HCl in 120 ml of 1 M sodium hydroxide solution and then diluting to 1000 ml with water. Subsequently,
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this NH2OH–HCl solution and 0.5 g oxidized fibers were placed in a 250 ml round-bottom flask connected by a
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manifold and with a stopcock for evacuation using a water pump. Then the stopcock was closed and the mixture was
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reacted by heating at 50 ºC for 2 h. When cooling to room temperature, a 25 ml aliquot of the reaction supernatant
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was taken to titrate with a 0.1 M HCl solution to pH 3.2. Using a 25 ml hydroxylamine hydrochloride solution as a
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blank titration was conducted in the same manner. The carbonyl content in the oxidized fibers was calculated using
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the equation.
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0.1M × (V2 − V1 ) × MWCO × 100% , m
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Carbonyl content (% w/w) =
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where 0.1 M is the concentration of HCl, V1 and V2 are the volumes (ml) of 0.1 M HCl consumed in titrations of
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blank and sample solutions, respectively, m is the dry weight of oxidized sample (mg).
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2.3.5. Ketone groups analysis
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from the carbonyl content confirmed as described above.
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2.3.6. Determination of fineness of oxidized bamboo pulp fibers
Fineness of the oxidized fibers in tex was measured as per standard method (SRPS F.S2.212., 1963) by dividing the mass of fibers by their known length.
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2.3.7. Morphology analysis
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Scanning electron microscopy (SEM) was used to observe longitudinal surface morphology. The samples were
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coated with gold. The instrument was a Japan JEOL JSM-6700F field emission-type scanning electron microscope
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with an accelerating voltage of 5 kV.
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2.3.8. Solid state 13C-CP/MAS NMR analysis
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Solid state 13C-NMR spectra of the oxidized bamboo pulp fibers were recorded using a Bruker AVANCE III-400
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(100.4 MHz) superconducting nuclear magnetic resonance spectrometer (Switzerland) with cross polarization and
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magic angle sample spinning under the following conditions: spinning rate 5 kHz, pulse delay 3 ms, contact time 5
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ms and compensate time 20 ms.
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2.3.9. FT-IR spectroscopy
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The Fourier transform infrared (FT-IR) spectra of samples were obtained on a Nicolet NEXUS-870 FT-IR
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spectrophotometer (Madison, USA) using KBr pellets, and samples were analyzed in the range of wavelength from
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4000 to 400 cm–1 with the result of 24 scans at the resolution of 4 cm–1.
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2.3.10. X-ray diffraction analysis
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X-ray diffraction (XRD) patterns of the bamboo pulp fibers were performed by the reflection method using a -6-
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RINT 2027 X-ray detector diffraction system (Rigaku Corporation, Tokyo, Japan) with a Cu Kα target and β Ni
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filter at a voltage of 40 kV, a current of 30 mA, a scan rate of 2°/min and 2θ range of 5–45°. The crystallinity degree
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(Xc) and crystalline index (CrI) of the fibers sample were calculated by the method of peaks separation and Gaussian
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peak fit with the Peakfit data processing software (Ver. 4.12, Seasolve Software Inc., Framingham, MA, USA)
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according to Eqs. (3) and (4), respectively:
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X c (%) =
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CrI =
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where Sc is the sum of all crystal areas, and Sa is the amorphous area. I002 is the maximum intensity (in arbitrary units)
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of the 002 lattice diffraction and Iam is the diffraction intensity of amorphous peak at about 18° of 2θ.
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2.3.11. Thermal analysis
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Thermogravimetry (TG) and differential thermogravimetry (DTG) analysis were carried out on 4–6 mg samples
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using a SDT Q600 TG/DSC thermal analyzer (TA Instrument Corp., USA) at a heating rate of 10 ºC/min from 20 to
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800 ºC with a constant nitrogen flow of 100 ml/min.
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2.3.12. Mechanical properties
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The tensile breaking strength and elongation of bamboo pulp fibers were taken on a YG021A electronic single
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yarn strength tester (Ningbo Textile Instrument Factory, China). The gauge length of the yarn sample was 200 mm,
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and speed was 200 mm/min. The fiber samples were conditioned in a room at 20 ºC and a relative humidity of 65%
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for 48 h before measurement.
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3. Results and discussion
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3.1. Effect of volume ratios of HNO3 and H3PO4 on the oxidation of bamboo pulp fibers
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For obtaining monocarboxy bamboo pulp cellulose (seen in Scheme 1), the impacts of HNO3/H3PO4–NaNO2
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oxidation on bamboo pulp fibers were assessed by determining the carboxyl group content, weight loss, fiber
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fineness and mechanical properties of fiber samples. In general, since commercial HNO3 contains small amounts of -7-
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nitrogen oxides including NO, NO2, N2O3 N2O5, etc., the oxidation of polysaccharide compounds by HNO3 alone is
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sluggish and frequently requires a catalyst (e.g. H2SO4), an initiator (e.g. NO2 or HNO2), and/or heating at high
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temperature (Ogata, 1978). In this study, the HNO3/H3PO4–NaNO2 mixture system oxidation of regenerated
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cellulose was investigated. The carboxyl content and weight loss of bamboo pulp cellulose oxidized with different
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ratios of HNO3 and H3PO4 at ambient temperature for 6 h are summarized in Table 1. The data presented in Table 1
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display that the ratio of HNO3 and H3PO4 used in the reaction did not have a significant effect on the oxidation of
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bamboo pulp fibers, and the HNO3/H3PO4–NaNO2 oxidation with the ratio of 2:1 (HNO3:H3PO4) obtained the
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maximum carboxyl content of 9.71% and a low weight loss of 17.02%. For comparison purpose, the result of the
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oxidation conducted using the HNO3/H2SO4–NaNO2 reaction system reported by Wanleg (1956) is also presented
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in Table 1. The oxidation with a mixture of HNO3/H2SO4–NaNO2 produced the oxycellulose with 8.75% carboxyl
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content, whereas the weight loss of oxidized bamboo pulp cellulose was found to be very high (about 56.92%). An
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addition in concentration of H2SO4 to speed the oxidation rate would cause further degradation of cellulose and
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hence a further decrease in the yield of oxycellulose (Wanleg, 1956). As shown in Table 1, the reaction system of
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HNO3 in combination with H3PO4 and NaNO2 can achieve the oxidized bamboo pulp fibers with a high carboxyl
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content (7.93–9.71%) and in a relatively low weight loss (15.29–19.03%). These results reveal that the
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HNO3/H3PO4–NaNO2 mixture produced oxycellulose in high yields due to slight hydrolysis of cellulose by a weak
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acid of H3PO4.
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As seen in Table 1, the HNO3/H3PO4–NaNO2 oxidized bamboo pulp celluloses corresponding to different ratios
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of HNO3 and H3PO4 contained 7.93, 8.82, 9.71, 8.48, 8.96, and 8.64% carboxyl contents, respectively. The amounts
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of carbonyl groups determined in these oxidized fibers were 1.83, 2.17, 2.69, 2.08, 2.26, and 2.13%, while the
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carboxyl contents in the samples, after further oxidation with sodium chlorite, corresponded to 7.91, 8.80, 9.71,
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8.47, 8.96, and 8.63%, respectively. These data illustrate that the oxycellulose by HNO3/H3PO4–NaNO2 mediated
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oxidation contained no aldehyde groups and the carbonyl content formed in the oxidized samples was only due to
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ketone groups. Simultaneously the HNO3/H2SO4–NaNO2 mixture oxidized bamboo pulp fibers separately
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contained 8.75% carboxyl content and 3.12% ketone content, and the analysis of carboxyl content in the oxidized
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fibers with sodium chlorite treatment indicated no aldehyde groups. According to the oxidation mechanism for
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alcohols reported by Ogata (1978) the nitrogen oxides have been broadly implicated as the oxidants in the oxidation
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of polysaccharide compounds by HNO3, solely or in the presence of an acid and NaNO2, furthermore, several
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different pathways may cause the generation of these species of nitrogen oxides in situ. During the
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HNO3/H3PO4–NaNO2 mediated oxidation, this reaction is initiated via abstracting a α-hydrogen atom of C6 site
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from cellulose by such odd-electron species of NO2 and NO, giving the product of Cell–C(·)H–OH, further attack
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by NO2 · and subsequent release of NO · and HNO2 forms Cell–CHO and Cell–CH(OH)2 intermediates, respectively.
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Following an attack of NO2 · may lead to a hydrogen abstraction reaction from the Cell–CHO and subsequent
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hydrolysis of the addition product, to yield the Cell–COOH. In the case of Cell–CH(OH)2, it is likely to undergo an
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initial hydrogen abstraction reaction followed by an attack by NO2 · and subsequent elimination of HNO2, resulting
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in the Cell–COOH formation of monocarboxy cellulose.
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3.2. Effects of reaction time and sodium nitrite concentration on the oxidation of bamboo pulp fibers
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The rate of carboxyl group generation in bamboo pulp cellulose by the oxidation from 0.7% and 1.4% sodium
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nitrite in combination with HNO3 and H3PO4 is shown in Fig. 1a. The oxidized bamboo pulp fibers exhibited
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increase in carboxyl content ranging from 0.95% to 10.38% with the increase of oxidation time and concentration of
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NaNO2. During the initial 60 min, the values of carboxyl group content in fibers oxidized by HNO3/H3PO4–NaNO2
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mixture using different concentrations of NaNO2 are very similar, and the reaction rate is fast. Subsequently
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increasing the reaction time, from 60 to 360 min, the HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers had higher
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carboxyl content compared to the fibers oxidized in the first stage, and the bamboo pulp fibers oxidized with 1.4%
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NaNO2 presented higher carboxyl content than the fibers oxidized with 0.7% NaNO2. Finally extending the
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oxidation time up to 480 min, caused only a small increase in the carboxyl content. The oxidation rate gradually
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decreased and the carboxyl content of bamboo pulp fibers oxidized with 0.7% and 1.4% NaNO2 became closer.
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During the HNO3/H3PO4–NaNO2 reaction, bamboo pulp cellulose chain-molecules would undergo scission with
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the production of soluble fragments due to side reactions of oxidation. The weight loss of oxidized bamboo pulp
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fibers may reflect not only the formation of soluble fragments but also the oxidative extent of the fibers oxidized by
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HNO3/H3PO4–NaNO2, as it is shown in Fig. 1b. The loss in weight of oxidized bamboo pulp fibers increased with
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the extended oxidation time, especially pronounced when the time of oxidation was over 60 min. Furthermore, the
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weight loss with values up to ~25.26% of the fibers oxidized by higher sodium nitrite charge (1.4% NaNO2)
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combining with HNO3 and H3PO4 is more noticeable than that (ranging in 3.48–15.72%) of the cellulose fibers
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oxidized with low sodium nitrite charge. In addition, the bamboo pulp fibers contraction phenomenon can be
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observed in the oxidation process of HNO3/H3PO4–NaNO2 mixture. Fig. 1c clarifies that the fineness of oxidized
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bamboo pulp fibers basically increased with higher NaNO2 concentration and increasing reaction time, and the -9-
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value of the oxidized fibers fineness was enhanced up to 13.81%, namely from 37.59 tex for starting (non-oxidized)
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fibers to 42.78 tex for the fibers oxidized by 1.4% NaNO2 during 480 min. Theoretically, increasing oxidation time may increase the carboxyl content of the oxidized bamboo pulp fibers
244
during the HNO3/H3PO4–NaNO2 mediated oxidation process, and high carboxyl content may subsequently enhance
245
the ability of further chemical modification for the oxidized fibers. However, the oxidation can break to some extent
246
the crystal structure in the original cellulose (Saito & Isogai, 2004). Figs. 1d and e show the effect of the oxidation
247
time on the mechanical properties of the oxidized fibers. In contrast with the starting bamboo pulp fibers, the
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breaking strength and elongation of oxidized fibers decreased with prolonging time. The tensile strength and
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elongation of the oxidized fibers did not change remarkably for the oxidation time in the range of 0–60 min,
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whereas it dramatically decreased when the oxidation time was over 60 min. The bamboo pulp fibers oxidized by
251
1.4% NaNO2 had much lower tensile strength and elongation than the fibers oxidized by 0.7% NaNO2 during the
252
severe oxidation degree, and the fibers oxidized with higher sodium nitrite concentration (1.4% NaNO2) and longer
253
oxidation time (480 min) became so rigid and crisp that their tensile strength and elongation could not be surveyed,
254
consequently, no data are shown in Figs. 1d and e. From these results it is clear that slight oxidation of
255
HNO3/H3PO4–NaNO2 for 60 min with a ratio of 2:1 (v/v) of nitric acid and phosphoric acid containing 1.4% (w/v)
256
of sodium nitrite yields adequate carboxyl content in the oxidized cellulose to ensure larger reaction activity for the
257
oxidized fibers when applied in more potential chemical modification. Meanwhile, the tensile properties of the
258
bamboo pulp fibers with slight oxidation had no significant change, which are suitable for dry goods and medical
259
textile applications.
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3.3. Morphology analysis of oxidized bamboo pulp fibers
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These micrographs obtained accordingly from the starting bamboo pulp fiber, the oxidized fibers with different
262
oxidation time are shown in Fig. 2. It can be seen from Fig. 2a that the non-oxidized bamboo pulp fiber consists of
263
fibrils which is basically arranged along the length of the fiber. There are full of grooves between fibrils in the
264
longitudinal fiber surface and apertures along the length of the fibrils, and its morphology is straight stripes with a
265
sleek shape. Figs. 2b and c show that bamboo pulp fibers were suffered from mild corrosion of oxidant in the
266
oxidation process of short time, some grooves on longitudinal surface of the oxidized fiber became deeper and
267
wider, but the whole surface morphology is almost similar to the original fiber.
268
Further extending the oxidation time, the HNO3/H3PO4–NaNO2 system oxidized fibers have a great different - 10 -
Page 10 of 30
longitudinal morphology as shown in Figs. 2(d, e and f) in comparison with non-oxidized ones. In Fig. 2d, the
270
oxidized fiber surface is very rough, and many narrow lines and apertures affected by oxidation can be easily seen,
271
some fibers start to generate slight convolution. The longitudinal surface of the bamboo pulp fiber with high
272
oxidation degree of long time is tightly contracted and crimpled to some extent as shown in Fig. 2e. So it could be
273
seen the grooves and fibril stripe outlines by spells. In the same way, Fig. 2f also represents the surface
274
characteristics like in Fig. 2e, but there is an addition in large extent, and some small crevices and holes appear on
275
the partial surface of the oxidized fiber. So there is nearly no sign of grooves between fibrils and the smooth and
276
sleek straight stripes become illegible. This indicates that the acute oxidation of long time by HNO3/H3PO4–NaNO2
277
mixture may result in the increase of weakness on the fiber surface, that are responsible for a decrease in tensile
278
properties of the oxidized bamboo pulp fibers.
279
3.4. FT-IR characterization
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Fig. 3 shows FT-IR spectra of non-oxidized bamboo pulp fiber and oxidized fibers with different oxidation time.
281
All of them had a broad peak corresponding to –OH group stretching of cellulose observed in the range 3100–3500
282
cm–1. However, these absorption peaks of –OH stretching in oxidized fibers gradually became narrower and
283
transferred to higher wavenumber position when increased the oxidation time. One possible reason for this could be
284
that the hydrogen bonding between cellulose chains was weakened, loosening the crystalline structure of fibers due
285
to deep oxidation for long time. For the bamboo pulp fibers oxidized for different oxidation time, a sharp band
286
associated with –OH group in-plane bending of cellulose at 1377.1 cm–1, a peak at around 1161.2 cm–1 due to
287
C–O–C asymmetric stretching and a little peak at 1030.2 cm–1 due to C–O stretching totally decreased somewhat in
288
the intensity. But another sharp band corresponding to –CH group bending of cellulose at about 1284.5 cm–1 was
289
very stronger than that of original bamboo pulp fiber. Compared with non-oxidized bamboo pulp fiber, the infrared
290
spectra of oxidized samples in Figs. 3(b–e) exhibit that a sharp characteristic absorption peak corresponding to C=O
291
stretching vibration of carbonyl group clearly appeared at 1739.7 cm–1 or so, whose absorptivity enhanced with
292
extending oxidation time, consistent with the increase of the carboxyl and ketone groups in oxidized fibers. This
293
should be attributed to the formation of carboxyl and carbonyl groups in the bamboo pulp fibers with
294
HNO3/H3PO4–NaNO2 mediated oxidation.
295
3.5. X-ray diffraction analysis
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Fig. 4 shows the X-ray diffraction spectra of bamboo pulp fiber samples treated with different oxidation time.
297
The diffraction patterns of oxidized fibers are similitude with that of the non-oxidized bamboo pulp fiber. It is well
298
known that the cellulose I structure of the native cellulose is changed to the cellulose II structure by cellulose
299
regeneration. Bamboo pulp fiber was regenerated one, so it was the cellulose II structure (Yang et al., 2008). The
300
locations of the characteristic peaks of bamboo pulp cellulose were obtained by deconvoluting the XRD spectra
301
with the Peakfit software. The diffractive peaks of oxidized bamboo pulp fibers at 2θ values of 12.4º and 20.2º were
302
respectively attributed to the (110) and (002) planes of the typical cellulose II structure. Hence it can be deduced
303
that the crystalline forms of the bamboo pulp fibers did not markedly change in the processing of
304
HNO3/H3PO4–NaNO2 mediated oxidation.
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Additionally, the crystallinity and crystalline index were calculated by foregoing Eqs. (3) and (4) after the
306
wide-angle X-ray diffraction (WAXD) patterns were deconvoluted by using the Gaussian deconvolution method
307
and the diffraction peaks numerical fitting ratio achieved 99.99% through the PeakFit software, and the results were
308
listed in Table 2. The data exhibited that the crystallinity and crystalline index of bamboo pulp fibers progressively
309
decreased with increasing oxidation time. This reason may be that the oxidation reaction of HNO3/H3PO4–NaNO2
310
mixture destroyed hydrogen bonding between bamboo pulp cellulose chains in both crystal and amorphous regions
311
during oxidizing in the range of our experiment conditions, which affects its physical properties, and that is
312
consistent with the above results from mechanical properties analysis of oxidized bamboo pulp fibers.
313
3.6. Solid-state 13C-NMR analysis of HNO3/H3PO4–NaNO2 oxidized cellulose
Ac ce p
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314
In HNO3/H3PO4–NaNO2 mediated oxidation of cellulose, the resonance signals of carbons in pyranose units of
315
bamboo pulp cellulose can be monitored by the solid-state CP/MAS 13C-NMR method. It can be comprehended
316
from Fig. 5 that the original and HNO3/H3PO4–NaNO2 oxidized bamboo pulp celluloses had broad C1,
317
non-crystalline C4, crystalline C6 and C2,3,5 signals, while small signal due to crystalline C4 and non-crystalline
318
C6 of cellulose II remained as well. The apparent C1 signal of the original bamboo pulp fiber at 102–108 ppm was
319
basically unchanged. The overlapped signals due to C2, C3 and C5 appeared at 69–81 ppm. The chemical shift and
320
intensities of C2, C3 and C5 signals somewhat changed by the oxidation. The signals for crystalline and
321
non-crystalline C4 carbons were located at 87–92 and 81–87 ppm, respectively (Isogai et al., 1989). The shape and
322
ratio of crystalline and non-crystalline C4 signals of the original bamboo pulp cellulose were roughly unchanged
323
even after the HNO3/H3PO4–NaNO2 oxidation for 6 h. These results show that the HNO3/H3PO4–NaNO2 mediated - 12 -
Page 12 of 30
324
oxidation of bamboo pulp fibers has nearly no influences on the chemical shift and the patterns of C1 and C4 in
325
solid-state
326
60–69 ppm, and the signal tops for crystalline and non-crystalline C6 carbons were respectively located at 64 and 61
327
ppm (Kumar & Kothari, 1999), the crystalline C6 signal gradually decreased with increasing the
328
HNO3/H3PO4–NaNO2 oxidation time, while the shoulder signal due to non-crystalline C6 carbons at 61 ppm
329
enhanced to some extent. Meanwhile, the signals due to carboxyl carbons in the oxidized fibers appeared at around
330
175 ppm, and their intensities increased with extended oxidation time. Probably the carboxyl groups formed in the
331
HNO3/H3PO4–NaNO2 oxidized bamboo pulp celluloses are mostly present at the C6 position, and the
332
HNO3/H3PO4–NaNO2 mediated oxidation at the C6 position primary hydroxyls are progressed in the
333
non-crystalline region and crystal zone of cellulose II of bamboo pulp fibers, in coincidence with those from XRD
334
analysis.
335
3.7. Thermal analysis
C-NMR spectra. On the other hand, the C6 resonance signal in bamboo pulp cellulose appeared at
M
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13
Fig. 6 shows the TG (a) and DTG (b) curves of the original bamboo pulp fiber and oxidized bamboo pulp fibers,
337
and the temperatures at which the major weight losses are also exhibited in Table 3. It was observed that all bamboo
338
pulp fiber samples showed a weight loss from 45 °C to 125 ºC due to the moisture content vaporization, and the
339
moisture content in these celluloses increased with adding oxidation extent (seen in Table 3). This is because the
340
bamboo pulp fibers with higher carboxyl content had the stronger affinity for water molecules. In the initial stage of
341
weight loss (180–380°C), almost all oxidized fiber samples started to decompose at lower temperatures than the
342
starting bamboo pulp cellulose, and the fastest thermo-decomposition temperature for the original fiber was 340 °C,
343
while they decreased to 235–318 °C for the oxidized products, again reflecting their lower crystallinity compared
344
with the original cellulose. Furthermore, the decomposition patterns became more complex with extending
345
oxidation degree, and two-stage decompositions occurred for all oxidized bamboo pulp materials during initial
346
weight losses (up to about 380 °C). The final stage was that the crystal region of the fibers had been completely
347
destructed and the cellulose decomposed totally from 380 ºC to the termination of 800 ºC. The final weight loss of
348
all oxidized fiber samples was in the 72–79% range, while for the starting bamboo pulp fiber the value was 86%.
349
Although the TG curves of the original bamboo pulp fiber and the oxidized fiber series are similar in some way, the
350
temperatures of the oxidized bamboo pulp fibers at the beginning degradation and maximal thermo-decomposition
351
rate are lower than those of the starting bamboo pulp fiber, so the thermal stability of these oxidized bamboo pulp
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352
fibers decreased with enhancing oxidation extent.
353
4. Conclusions The results presented in this study show that the HNO3/H3PO4–NaNO2 mediated oxidation of bamboo pulp fibers
355
with different sodium nitrite concentrations and reaction time can obtain oxidized fiber products with high yield and
356
various carboxyl content. It is manifested that the different ratios of HNO3 and H3PO4 used in the oxidation reaction
357
did not significantly influence the weight loss and carboxyl content of oxidized bamboo pulp fibers. Since the
358
carboxyl content and weight loss of the oxidized fiber increased with adding oxidation time, we can easily acquire
359
the oxidized bamboo pulp fiber with expectant yield and carboxyl content by controlling the oxidation time of
360
HNO3/H3PO4–NaNO2 mixture. With increasing reaction time, the fiber fineness increased and mechanical
361
properties of the oxidized bamboo pulp fiber reduced on the whole.
an
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354
From the analysis of structure, it can be deduced that the crystallinity of the oxidized bamboo pulp cellulose
363
decreased gradually with increasing oxidation extent, but the typical cellulose II structure was not changed. The
364
results from TG and DTG show that the oxidized fiber materials had a lower thermal stability than the original
365
bamboo pulp fiber. Solid-state 13C-NMR and X-ray diffraction analyses revealed that the carboxyl and carbonyl
366
groups introduced into the HNO3/H3PO4–NaNO2 oxidized celluloses were mostly present at the C6 position of
367
amorphous and crystalline regions in bamboo pulp fibers. In addition, the tensile properties of oxidized fibers were
368
not changed very much, and the oxidized fiber materials containing 1.13–3.56% of carboxyl content and with
369
94–98% of high yields can be obtained in the range of oxidation time from 15 to 60 min. It can be implied that the
370
resultant oxidized bamboo pulp fibers possess more potential for still more chemical modification due to their
371
greater active carboxyl and carbonyl groups and may find extensive applications in textile and medical industries.
372
Acknowledgements
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This research has been supported by Scientific Research Foundation for Returned Scholars of Ministry of
374
Education of China (Grant number (2011)1568), Anhui Provincial Natural Science Foundation of China (Grant
375
number 10040606Q16), and Anhui Provincial Higher Academy Natural Science Research Key Program of China
376
(Grant number KJ2012A116).
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Page 14 of 30
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Kumar, V., & Yang, T. (2002). HNO3/H3PO4–NaNO2 mediated oxidation of cellulose-preparation and
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nitrogen dioxide/carbon tetrachloride. Fibers and Polymers, 13(5), 576–581. Yackel, E. C., & Kenyon, W. O. (1942). The oxidation of cellulose by nitrogen dioxide. Journal of the American
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Figures
445
Scheme 1. HNO3/H3PO4–NaNO2 mediated oxidation of bamboo pulp cellulose.
446 447 448
Fig. 1. Relationship between the carboxyl group content (a), weight loss (b), fiber fineness (c), breaking strength (d) and breaking elongation (e) of the oxidized bamboo pulp fibers and the oxidation time and the concentration of NaNO2 used for the oxidation.
449 450
Fig. 2. SEM micrographs of: (a) non-oxidized bamboo pulp fiber, (b) fiber oxidized for 15 min, (c) fiber oxidized for 60 min, (d) fiber oxidized for 180 min, (e) fiber oxidized for 300 min, and (f) fiber oxidized for 360 min.
451 452
Fig. 3. FT-IR spectra of: (a) starting bamboo pulp fiber, (b) fiber oxidized for 60 min, (c) fiber oxidized for 180 min, (d) fiber oxidized for 300 min, and (e) fiber oxidized for 360 min.
453 454
Fig. 4. X-ray diffraction patterns of bamboo pulp fibers oxidized with different oxidation time: (a) 0 min, (b) 60 min, (c) 180 min, (d) 300 min, and (e) 360 min.
455 456
Fig. 5. Solid-state CP/MAS 13C-NMR spectra of the original bamboo pulp fiber and HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers under various conditions.
457
Fig. 6. TG (a) and DTG (b) curves of the starting bamboo pulp fiber and a series of oxidized bamboo pulp fibers.
us
cr
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444
an
458 459 460 461
M
462 463
467 468 469 470 471 472 473 474 475 476
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466
Ac ce p
465
d
464
477 478 479 480 481 482 483 484 485 - 18 -
Page 18 of 30
cr
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d
M
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Scheme 1. HNO3/H3PO4–NaNO2 mediated oxidation of bamboo pulp cellulose.
Ac ce p
493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531
ip t
486 487 488 489 490 491 492
- 19 -
Page 19 of 30
532 533
a 12 10
1.4% NaNO2
6 4
20 15
ip t
8
10
cr
5
2
0
0
0.7% NaNO2 1.4% NaNO2
420 410
M
400
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
390 380
d
370 360
te
Fineness of oxidized fibers (dtex)
c 430
0
us
534 535
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
an
0
0
d 1.6
e 16
0.7% NaNO2
0.7% NaNO2
14
1.4% NaNO2
1.4
Tensile elongation (%)
Tensile strength (cN/dtex)
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
Ac ce p
536 537
1.2 1.0 0.8 0.6 0.4
1.4% NaNO2
12 10 8 6 4 2
0.2
0
0.0 0
538 539 540 541 542 543 544 545 546 547
0.7% NaNO2
1.4% NaNO2
Weight loss (%)
Carboxyl content (%)
b 25
0.7% NaNO2
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
0
50 100 150 200 250 300 350 400 450 500 Oxidation time (min)
Fig. 1. Relationship between the carboxyl group content (a), weight loss (b), fiber fineness (c), breaking strength (d) and breaking elongation (e) of the oxidized bamboo pulp fibers and the oxidation time and the concentration of NaNO2 used for the oxidation.
- 20 -
Page 20 of 30
a
b
c
d
e
557 558 559 560 561 562 563 564 565 566 567 568 569 570 571
us
f
Ac ce p
te
555 556
d
M
an
553 554
cr
ip t
548 549 550 551 552
Fig. 2. SEM micrographs of: (a) non-oxidized bamboo pulp fiber, (b) fiber oxidized for 15 min, (c) fiber oxidized for 60 min, (d) fiber oxidized for 180 min, (e) fiber oxidized for 300 min, and (f) fiber oxidized for 360 min.
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Page 21 of 30
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Fig. 3. FT-IR spectra of: (a) starting bamboo pulp fiber, (b) fiber oxidized for 60 min, (c) fiber oxidized for 180 min, (d) fiber oxidized for 300 min, and (e) fiber oxidized for 360 min.
Ac ce p
578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610
an
us
cr
ip t
572 573 574 575 576 577
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Page 22 of 30
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Fig. 4. X-ray diffraction patterns of bamboo pulp fibers oxidized with different oxidation time: (a) 0 min, (b) 60 min, (c) 180 min, (d) 300 min, and (e) 360 min.
Ac ce p
617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649
an
us
cr
ip t
611 612 613 614 615 616
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Page 23 of 30
te
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Fig. 5. Solid-state CP/MAS 13C-NMR spectra of the original bamboo pulp fiber and HNO3/H3PO4–NaNO2 oxidized bamboo pulp fibers under various conditions.
Ac ce p
656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688
an
us
cr
ip t
650 651 652 653 654 655
- 24 -
Page 24 of 30
689 690 691 692 693 694
a
an
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Fig. 6. TG (a) and DTG (b) curves of the starting bamboo pulp fiber and a series of oxidized bamboo pulp fibers.
Ac ce p
695 696 697 698 699
us
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b
- 25 -
Page 25 of 30
Tables
700 701
Table 1 Effect of different ratios of HNO3 and H3PO4 (or H2SO4) on oxidation of bamboo pulp cellulose
702 703 704
Table 2 Characteristic peaks and crystalline parameters of bamboo pulp fibers oxidized with different oxidation time obtained from WAXD.
705 706
Table 3 Thermogravimetric data on the starting bamboo pulp fiber and oxidized bamboo pulp fiber samples
cr us an M d te Ac ce p
707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754
ip t
699
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Page 26 of 30
Table 1 Effect of different ratios of HNO3 and H3PO4 (or H2SO4) on oxidation of bamboo pulp cellulose Carboxyl contentb (%)
Carbonyl contentc (%)
Oxidation system: HNO3/H3PO4–NaNO2d A 11:1 B 4:1 C 2:1 D 1:1 E 1:2 F 1:4
16.34 15.29 17.02 17.80 19.03 18.68
7.93 8.82 9.71 8.48 8.96 8.64
1.83 2.17 2.69 2.08 2.26 2.13
Oxidation system: HNO3/H2SO4–NaNO2 G 11:1e
56.92
8.75
a
3.12
us
Represents average value of 3 determinations, standard deviation (%) range ±0.68–1.19. Represents average value of 3 determinations, standard deviation (%) range ±0.14–0.47. c Represents average value of 3 determinations, standard deviation (%) range ±0.11–0.91. d All oxidations were proceeded at room temperature for 6 h with a bath ratio of 1:30 (w/v) of bamboo pulp fiber to acid solution and about 1.4% (w/v) of NaNO2. e Corresponds to the same ratio used by Wanleg (1956). The ratio of bamboo pulp fiber, acid mixture and NaNO2 was as same as the HNO3/H3PO4–NaNO2 reaction system.
te
d
M
an
b
Ac ce p
760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804
HNO3:H3PO4 (or H2SO4) (v/v)
ip t
Weight lossa (%)
Oxidation No.
cr
755 756 757 758 759
- 27 -
Page 27 of 30
Location of characteristic peak, 2θ (º)
a b c d e
12.62 12.43 12.36 12.30 12.51
16.56 16.47 16.29 16.72 16.34
20.28 20.25 20.34 20.12 20.14
22.02 22.16 22.10 22.34 22.27
Crystalline index (%)
44.98 40.43 38.21 30.69 29.14
52.24 49.68 45.97 32.41 28.03
d
M
an
us
cr
15.40 14.82 14.64 14.94 15.48
Crystallinity (%)
ip t
Sample No.
te
811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858
Table 2 Characteristic peaks and crystalline parameters of bamboo pulp fibers oxidized with different oxidation time obtained from WAXD.
Ac ce p
805 806 807 808 809 810
- 28 -
Page 28 of 30
Table 3 Thermogravimetric data on the starting bamboo pulp fiber and oxidized bamboo pulp fiber samples
Original bamboo pulp fiber 1 h-oxidation 3 h-oxidation 5 h-oxidation 6 h-oxidation
Final weight loss
Degradation temperaturea (°C)
Weight loss (%)
Degradation temperaturea (°C)
Weight loss (%)
10.78
278.8
63.67
369.5
85.62
13.22 14.58 16.76 17.31
236.7, 286.6 216.3, 263.2 194.4, 279.1 190.2, 266.3
55.40 56.49 60.57 59.21
363.3 371.2 386.7 378.9
75.54 72.01 79.08 78.27
d
M
an
us
Two or more temperatures indicate two or more stages of decomposition.
te
a
Initial weight loss
Ac ce p
864 865 866
Volatile content (%)
ip t
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Highlights H3PO4/HNO3–NaNO2 mediated oxidation of bamboo pulp cellulose obtains 6-carboxycellulose. Supermolecular structure of the oxidized cellulose was characterized. Correlations on structure and properties of bamboo pulp cellulose fibers during oxidation were investigated. Oxidation mostly occurred at C6 primary hydroxyls of non-crystalline and crystal regions of cellulose.
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