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Oxidation of HydroxyethylCellulose with SodiumHypochloriteMohamed Adel Yousef aa Helwan University, Faculty of Science, ChemicalDepartment , Helwan, Cairo, EgyptPublished online: 22 Sep 2006.

To cite this article: Mohamed Adel Yousef (1991) Oxidation of Hydroxyethyl Cellulosewith Sodium Hypochlorite, Polymer-Plastics Technology and Engineering, 30:4,413-423, DOI: 10.1080/03602559108021004

To link to this article: http://dx.doi.org/10.1080/03602559108021004

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P0LYM.-PLAST. TECHNOL. ENG., 30(4), 41 3-423 (1991)

OXIDATION OF HYDROXYETHYL CELLULOSE WITH SODIUM HYPOCHLORITE

MOHAMED ADEL YOUSEF

Helwan University Faculty of Science, Chemical Department Helwan, Cairo, Egypt

Abstract

Oxidation of water-soluble hydroxyethyl cellulose with sodium hypo- chlorite has b e a studied under a variety of conditions. Variables studied included the concentration of sodium hypochlorite, pH, temperature, and catalyst. It was found that the rate of oxidation was increased by increas- ing the hypochlorite concentration and/or reaction temperatures. The rate is very slow in an alkaline medium and very rapid in neutral medium. The effect of catalyst on the rate of oxidation was also investigated. The apparent viscosities of the oxidized samples at various rates of shear were also investigated.

INTRODUCTION

Cellulose is very sensitive toward oxidizing agents. All the functional groups in the building units of the cellulose molecules can readily be oxidized, such as: oxidation of the primary hydroxyl groups in the 6-position to aldehyde or carboxyl groups (Ivanov et al., 1958; Pavlyuchenko et al., 1960); oxidation of the secondary hydroxyl groups 2 and 3 to ketone groups; oxidative opening of rings to form two aldehyde or carboxyl groups; etc. (Davidson, 1941; Ott et al., 1955). According to the conditions of oxidation, the resulting products have either a marked acidic or an equally pronounced reducing character.

Many oxidizing agents are used. These include: chromic acid (Reinhardt et al., 1959, 1961), chromic acid-oxalic acid mixture (Reinhardt et al., 1958), periodic acid (Reinhardt et al., 1961; Meller, 1956), sodium metaperiodate

41 3

Copyright 0 1991 by Marcel Dekker, Inc.

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(Reinhardt et al., 1959), and sodium hypochlorite (Reinhardt et al., 1961) under a variety of conditions.

Hydroxyethyl cellulose (HEC) is one of the most important cellulose derivatives used as thickening agent in textiles (Okuyama and Yamada, 1977), adhesives (Dominguez, 1952), detergents (Baco and Smith, 1952), water treat- ment (Kemmer et al., 1952), cement (William et al., 1988), and ceramics (Paul, 1970) as well as the paper industry (Willi et al., 1982; Rabas et al., 1980). Nevertheless, no work has been published yet in the oxidation of water-soluble hydroxyethyl cellulose with sodium hypochlorite.

This work presents a detailed study of the action of sodium hypochlorite on water-soluble hydroxyethyl cellulose (HEC). It is believed that the results of this investigation would prove of value to the utilization of oxidized HEC in the adhesives and paper coating industries.

EXPERIMENTAL

Materials

Commercial water-soluble hydroxyethyl cellulose (Tylose 250 HHX) was kind- ly supplied by Hoechest, Germany. Sodium hypochlorite was prepared by pass- ing chlorine gas into sodium hydroxide solution (40 g/L).

Method of Oxidation

Oxidation of hydroxyethyl cellulose (HEC) with sodium hypochlorite was per- formed by introducing the HEC sample into a hypochlorite solution of known concentration and specific temperature, using a liquor ratio of 50: 1. The pH was adjusted to 7.4 using potassium dihydrogen phosphate as buffer. The mix- ture was shaken immediately and also occasionally during the course of oxidation.

Analysis

Chlorine consumption, expressed as milligrams of chlorine/gram HEC, was determined according to a method of Whistler (1963), which was modified as follows:

The decrease in hypochlorite concentration due to oxidation of HEC was determined from the difference between the concentration of “blank” solu- tion and the solution containing the HEC at the time of sample removal from the

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OXIDATION OF HYDROXYETHYL CELLULOSE 41 5

hypochlorite solution. Hypochlorite concentrations were determined by titrating portion (10 mL) of solution with 0.1 N sodium thiosulfate solution after the addition of 10 mL of iodate-free potassium iodide solution (10%) and 5 mL acetic acid.

The amount of chlorine consumed was calculated as follows:

(U - b) X V X N (35.5 X 1OOO) mg chlorine/g HEC =

10 x w x lo00

where: a = volume of thiosulfate consumed by 10 mL of NaOCl solu- tion of the “blank”

b = volume of thiosulfate consumed by 10 mL of the NaOCl solution

V = the bulk solution of the NaOCl used in oxidation of each sample

N = normality of sodium thiosulphate solution ( N / l O ) W = dry weight of HEC

Determination of Apparent Viscosity

The apparent viscosity at various rates of shear of the oxidized samples, in a concentration of 20%, were tested at 25 “C using a rotary viscometer (Haake Mess. Technik GmbH, U.Co., Germany).

RESULTS AND DISCUSSION

Oxidation of HEC was investigated under a variety of conditions. Variables studied were concentration of the oxidizing agent, pH of the oxidizing medium, and temperature of the reaction. The influences of MgC12 and ZnClz as catalysts of the oxidant were also investigated. The apparent viscosity of the oxidized samples at various rates of shear was also investigated.

Effect of Sodium Hypoc hlorite Concentration

To investigate the effect of sodium hypochlorite concentration on the rate of oxidation, HEC samples were oxidized according to the aforementioned ex- perimental procedure using different concentrations ranging from 1 to 4 g/L

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41 6 YOUSEF

active chlorine. The chlorine consumption was determined at different inter- vals of time and the results obtained are illustrated in Fig. 1.

It is clear from Fig. 1 that increasing the NaOCl concentration from 1 to 4 g/L active chlorine causes a considerable enhancement in the chlorine con- sumption, particularly in the latter stages of the reaction; that is, the rate of

a 1 g / t c ~ ~

1 80 b 2 g l t C I 2

FIG. 1. Rate of oxidation of HEC with NaOCl of different concentrations @H 7.4, at 25°C).

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OXIDATION OF HYDROXYETHYL CELLULOSE 41 7

oxidation is influenced significantly by the reaction time, being more pro- nounced at a higher NaOCl concentrations.

The significant enhancement in the chlorine consumption at higher concen- trations of sodium hypochlorite suggested that the chlorite ions follow several routes for decomposition, and that these routes are determined by the hypochlorite concentration. Among these routes, mention is made of the follow- ing (Ridje and Little, 1942; Burki and Schaof, 1921):

OC1- + H20 * HO- + HOCl HOCl + H20 + H30' + OC1- Clz + H20 + 2HOC1 - 2HC1 + 0 2

3HOC1 --* 2HC1 + HC103

HOCl + H+ + C1-

2c10- - 2c1- + 0 2

C10- + 2HOC1 - C 1 0 ~ + 2H+ + C1-

In addition, envolvement of the free radical chain in decomposition of sodium hypochlorite has been reported (Holst, 1954):

HClO + OC1- - C10' + C1- + HO' HO' + OC1- - C10' + HO- C10' + OC1- --* 2C1- + 0 2 + HO'

In combination with the above is the greater availability of hypochlorite ions in the vicinity of the molecular structure of cellulose at higher sodium hypochlorite concentration and their better opportunity to attack the immobilized HEC hydroxyls as well as the newly formed aldehydic groups together with glucosidic bonds. Stated in other words, beside accelerating its own decom- position during oxidation of HEC, the action of the decomposition products on the HEC proceeds faster by virtue of their greater availability in the prox- imity of the HEC at higher hypochlorite concentrations.

Effect of Reaction Temperature

In order to investigate the effect of temperature, the oxidation reaction was conducted at different temperatures ranging from 25 O to 75 "C, 2 g/L NaOCl solution of pH 7.4, at a liquor ratio of 50: 1. The results obtained are shown in Fig. 2.

It is clear from Fig. 2 that the temperature plays a dominant role in the rate of oxidation of HEC with NaOCl. The higher the temperature, the higher the

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Tmp.

Tmp.

Tmp.

75' c

50'C

25' c

I I 1 I I I I

20 40 60 80 100 120 140

OXIDATION TIME, minutes

FIG. 2. Effect of temperature on the rate of oxidation of HEC with NaOCl2 g/unit C1 and pH 7.4.

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OXIDATION OF HYDROXYETHYL CELLULOSE 419

chlorine consumption, specially at the early stage of the reaction. This indicates that the rate of oxidation increased in the order 75" > 50" > 25°C. This may be due to the acceleration of the aforementioned equations by increasing the reaction temperature.

It can be also observed that, at higher reaction temperature (i.e. 75 "C), even though the extent of oxidation increased significantly at the early stage of the reaction, it does slow down during the latter stage of the reaction. This could be ascribed to the reduction in both the hypochlorite concentration and the availability of sites for oxidation of HEC as the oxidation reaction proceeds.

Effect of pH

It has been reported (Reinhardt et al., 1961; Whistler, 1963; Clibbens and Ridge, 1927) that oxidation of cellulose with NaOCl reveals that both the rate of oxidation and the nature of the products are determined by the pH of the solution. In alkaline solution (pH 11 3, where the effective oxident is the hypochlorite ion, the reaction is very slow and gives rise to acidic oxycellulose of low reducing power. In mildly acidic solution (pH 3-5), in which the ef- fective oxidant is undissociated hypochlorous acid, the reaction is also rather slow, but it gives rise to highly reducing oxycellulose containing few acidic groups. In neutral solutions, which contain both hypochlorite ions and hypochlorous acid, the rate of oxidation is at its maximum and oxycelluloses of mixed types are produced.

Hence, it appears of interest to examine the influence of pH on the rate of oxidation of HEC with NaOCl. For this purpose, three parallel experiments were carried out at different pH values: 2.5, 7.5, and 11.5 adjusted with phosphoric acid, potassium dihydrogen phosphate, and sodium bicarbonate, respectively. The oxidation reaction was carried out in NaOCl solution con- taining 2 g/L active chlorine at 25 "C using a liquor ratio of 50: 1. The results obtained are shown in Fig. 3.

It is clear from the data that the pH has a remarkable effect on the rate of oxidation. In agreement with previous studies, that rate of oxidation is rapid at pH 7.5 (Reinhardt et al., 1961; Whistler, 1963).

Effect of Catalyst

To investigate the effect of catalyst on the rate of oxidation, three parallel ex- periments were carried out. In one, the oxidation was conducted as usual, whereas in the second and third, the reaction was undertaken in presence of 2 g/L of MgC12 and ZnC12, respectively. In every case, a concentration of

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420

80 -

70 - fi I m

3 0

60-

z g - - 2 40-

0 8

30- g A I

20-

10 -

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at pH 7.4

at pH 2.5

- at pH 11.5

0 0 20 40 60 00 100 . 120 140

OXIDATION TIME, minutes

FIG. 3. Rate of oxidation of HEC with NaOCl at different pH values (2 g/unit Clz at 25°C).

2 g/L active chlorine was used at 50 "C and a liquor ratio of 50: 1. The results are represented in Fig. 4.

It is clear from the data that the effect of the catalyst depends upon the time of oxidation. At the initial stages (20 min) the maximum chlorine consump- tion was obtained in the case using M g Q as catalyst, while at the end, (140 min) the opposite holds true.

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OXIDATION OF HYDROXYETHYL CELLULOSE

80

70

0 w I m 60- . cu - 0 m E 50- 2- 0 I- f 4 0 - 0 z s 3 5 20-

30- - IT

10

42 1

- a without catalyst.

b with ZnC12 as catalyst.

c with MgCI2 as catalyst. -

b

-

6 - 1 I I 1 1 I 1 . 0 20 40 60 80 100 120 140

FIG. 4. Effect of ZnClz and MgC12 on the rate of oxidation of HEC with NaOCl (2 g/unit C12, pH 2.5, at SOOC).

Apparent Viscosity

Since HEC is generally used as a viscious solution, it is of great interest to investigate its viscosity. Hence pastes from oxidized HEC of different chlorine consumption in a concentration of 2% were prepared in distilled water. The apparent viscosities of these pastes were tested as mentioned in the experimental section. The results obtained are given in Table 1.

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TABLE 1 Effect of the Degree of Oxidation, Expressed as Chlorine Consumption,

on the Apparent Viscosity of Hydroxyethyl Cellulose (HEC)p

Chlorine consumption of oxidized HEC (mg C12/1 g HEC) 129 258 387 516 645 774 903 1032 1161

Apparent viscosity in P a 4 X I d at various rates of shear:

6.5 15.5 19.5 28.5 44.0 76.5 97.0

113.5

24.3 21.5 20.1 18.4 17.5 16.3 13.5 12.8 12.5 19.1 17.4 16.2 15.1 14.6 14.0 11.9 11.2 10.5 17.4 16.0 14.1 13.0 12.3 11.6 10.7 10.2 9.3 14.2 13.7 12.9 12.2 11.6 10.5 9.8 9.1 8.4 11.6 10.5 9.7 9.1 8 . 5 8.1 7.5 7.2 6.8 9.4 8.2 7.6 7.2 6.7 6.5 6.1 S . 8 5.6 8.7 7.9 7.1 6.7 6.4 5.9 5.4 5.2 4.9 7.2 6.5 6.0 5.4 5.3 5.1 5.0 4.8 4.7

OThe apparent viscosity was determined at 25°C at a concentration of 2% in distilled water.

It is clear from the data that the apparent viscosity depends on the degree of oxidation expressed as milligrams Clz/gram HEC. It decreases as the chlorine consumption increases. This holds true regardless of the rate of shear; for example, at a rate of shear of 516 S-' the apparent viscosity decreases from 18.4 to 5.4 P a 4 X 1@ as the chlorine consumption increases from 6.5 to 113.5 mg Clz/g HEC.

The decrease in the apparent viscosity by increasing the extent of oxidation reaction, expressed as chlorine consumption, may be due to glucosidic bond scission under the influence of the decomposition products of sodium hypochlorite. On the other hand, it is clear from the data that the apparent viscosity depends on the rate of shear. As the latter increases, the apparent viscosity decreases, which indicates that these pastes are non-Newtonian solutions.

Properties of the oxidized HEC and its utilization in the adhesives and coated paper industries will be published in a forthcoming paper.

CONCLUSION

It can be concluded from these investigations that the rate of oxidation of HEC with NaOCl increases by increasing the oxidizing agent concentration and/or reaction temperature. The rate also depends on the pH of the reaction medium.

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OXIDATION OF HYDROXYETHYL CELLULOSE 423

The apparent viscosity of the oxidized samples depends on both the chlorine consumption and the rate of shear.

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