The features of nitric acid mercerization of cellulose

Size: px

Start display at page:

Download "The features of nitric acid mercerization of cellulose"

Reynard Alfred Murphy
6 years ago
Views:

1 Cellulose 7: 57 66, Kluwer Academic Publishers. Printed in the Netherlands. The features of nitric acid mercerization of cellulose E. V. GERT, A. SOCARRAS MORALES, O. V. ZUBETS and F. N. KAPUTSKII Research Institute of Physico-Chemical Problems of the Belarussian State University, 14 Leningradskaya st., Minsk, Belarus, Received 18 June 1998; accepted 4 February 2000 Abstract. Transformation of native cellulose species into cellulose-ii polymorph through the additive Knecht compound formed under the action of 68.5% nitric acid has been studied. Probable causes of peculiar temperature effects in the course of phase transformations taking place in cellulose of various origin, crystallite dispersity, or morphologic structure are discussed. The processes of hydrolytic destruction and esterification of starting materials during their mercerization by this non-traditional agent at 20 Cand0 C are quantitatively characterized. In the case of mercerization of wood microcrystalline cellulose at 20 C, a decrystallizing effect due to side reactions of partial nitration is noted. Key words: cotton cellulose, wood cellulose, microcrystalline cellulose, nitric acid, Knecht compound, mercerization, decrystallization, temperature effects Introduction Practical importance of cellulose mercerization phenomenon based on intracrystallite swelling in alkali hydroxide solutions is well known. The intracrystallite swelling can also be caused by HNO 3 at concentrations close to 68.4%, the value corresponding to azeotrope composition. As a result of the swelling, crystalline phase of an additive compound, the Knecht compound, is formed. Although this compound has been known for a long time (Knecht, 1904), its features remain yet obscure under many aspects (Gert, 1997). Under the action of water, the cellulose II polymorph is regenerated from the Knecht compound: a behaviour similar to that of alkaline cellulose, which allows the term acid mercerization to be used. Examples of successful utilization of this version of mercerization for preparation of structurally and chemically modified species of cellulose in powder form are already available (Gert, 1996), in spite of insufficiency of information concerning features of cellulose phase transformations in nitric acid medium. Thus far, it was known only that the depth of phase transformation under the action of HNO 3 at room temperature is strongly dependent on parameters of crystalline structure of native cellulose (Gert, 1996; Ioelovich and Veveris,

2 ). The greatest susceptibility towards structural reorganizations is noted for wood cellulose which is characterized by moderate crystallinity (Gert, 1996). There are publications in the literature (Ioelovich, 1991; Ioelovich and Veveris, 1991) where mean transversal dimensions of crystallites ( L) are considered to be the main factor determining the depth of phase transformations associated with formation of both the alkaline cellulose and the Knecht compound. In this paper, effects of temperature on the process of nitric acid mercerization of cellulose of different origin and morphological structure are discussed. Chemical and physical changes taking place in the starting materials concomitantly with the polymorphic transformation caused by the nontraditional mercerizing agent are assessed. Experimental The principal starting materials were cotton cellulose ( Hercules brand, 99% α-cellulose; DP = 2000), sulphite cellulose of southern pinewood ( Florainier brand, 96% α-cellulose; DP = 1200), and microcrystalline forms of these cellulose species (DP = 200 and 160, respectively). Preparation of cotton and wood microcrystalline cellulose (MCC) included the following operations: boiling the fibres with 8% HNO 3 (1 h, 20 ml/g), multiple washing the dispersion with water, displacement of water by acetone, drying at room temperature, trituration in a mortar, separation of a fraction passing through the 100 µm sieve. Crystallinity indices determined by the method reported by Ioelovich and Veveris (1983) were 0.70 and 0.72 for cotton cellulose and MCC, and 0.63 and 0.69 for wood cellulose and MCC, respectively. Purified and bleached flax fibres (97% α-cellulose), as well as rayon fibres free from technological additives were also used. Treatment of the named materials was preceded by conditioning in a desiccator over saturated NaNO 2 solution (relative humidity 66%). Commercial 72% HNO 3, highest purity brand, was used to prepare the 68.5% solution. The acid concentration was controlled by density measurements at 20 C. Sample mercerization conditions were: liquor ratio 20 ml/g, reaction temperatures 20 C (thermostat) and 0 C (a vessel with melting ice). Washing sequence and drying conditions were the same as in the MCC preparation procedure. The degree of polymorphic transformations was determined by the method described by Ioelovich and Veveris (1983) from X-ray patterns obtained with diffractometer model HZG-4a (CuK α radiation, Ni-filter). Flat disc samples were prepared by the press-out technique. Mean values were taken from 3 4 parallel measurements. The measurement error was ±5%.

3 For comparative evaluation of the swelling degree, sections of the same fibre (flax or rayon) were used. Two samples each underwent swelling in either alkaline or acid solution, the third served as a control. The swollen and the control fibre sections were then laid down side by side onto an object glass, and photographs were taken using a Biolam I microscope. The degree of polymerization of cellulose was determined from viscosities of the respective cellulose nitrate solutions, according to the method of Nikitin (1962), which has been successfully tested using samples with known DP. Bound nitrogen content of the samples was determined by the Kjeldahl method (Cheronis and Ma, 1964). 59 Results and discussion In contrast to alkali hydroxides, HNO 3 is a multifunctional reagent with respect to cellulose. It is capable of nitrating, oxidative, and hydrolysing actions. This circumstance should be taken into account when selecting nitric acid mercerization conditions. For example, the HNO 3 capability to form the Knecht compound with cellulose is clearly manifested already at a concentration of 68% (Gess, 1928), and this tendency increases up to the concentration of 75%. At the same time, from the concentration of 69% on, an accumulation of the HNO 3 pseudo form (HO NO 2 ) occurs which provokes cellulose nitration (Nikitin, 1962; Warwicker, 1971). Hence, the stimulation of phase transformation by increasing the HNO 3 solution concentration is complicated by nitrating ability of the acid at concentrations above 69%. Taking this into account, 68.5% HNO 3 was used. Lower temperatures are known to favour traditional caustic mercerization of cellulose. This is usually explained by differences in temperature coefficients for the formation rate and hydrolysis rate of alkali soda cellulose (the latter value is greater). The temperature dependence of nitric acid mercerization was not investigated earlier. It was mentioned only in Knecht s original publication (Knecht, 1904) that a decrease in temperature leads to an increase in swelling degree of Ramie cellulose in 68.6% HNO 3. According to these data, the swelling is accomplished rapidly, being completed within 2min. The experimental results obtained in our studies (Figure 1) give evidence of a peculiar effect of temperature on nitric acid mercerization process occurring in cellulose of different origin and morphology. When cotton cellulose is taken as the starting material, the temperature dependence is similar to that observed for traditional mercerization, i.e. temperature decrease from 20 C to 0 C stimulates the polymorphic transformation. However, the treatment conditions chosen do not assure a complete transformation of the starting material into the polymorph II. Phase transformations are substantially slower

4 60 Figure 1. Degree of transformation into polymorph-ii of fibrous (a) and microcrystalline (b) forms of wood (1,2) and cotton (3,4) cellulose as function of time of interaction with 68.5% HNO 3 at 20 C (1,4) and 0 C (2,3). in cotton MCC than in initial cellulose (cf. curves 3 and 4, Figures 1a, b). The superiority of fibrous cotton cellulose compared with MCC in ability to form the Knecht compound was also noted earlier (Gert et al., 1990; Ioelovich and Veveris, 1991). It was suggested that amorphous regions fulfil a mediator function, facilitating the HNO 3 access to crystallites. However, this viewpoint does not provide an explanation of the following observations. The above conditions of interaction with HNO 3 being applied to wood sulphite cellulose cause its complete and relatively rapid transformation into the polymorph-ii. In contrast with cotton cellulose, this process occurs more rapidly at 20 C, not at 0 C. The anomalous character of temperature dependence is more pronounced in samples with a conventional (amorphocrystalline) morphology (Figure 1a). This observation is confirmed in experiments with wood cellulose of different brand names. Wood MCC, in contrast with cotton MCC, is mercerized at an unexpectedly high and almost temperature-independent rate, up to a conversion degree of 60%. (Figure 1b). It is at the final stage only that a somewhat slower polymorphic transformation is observed at 0 C compared with that at 20 C. From the data obtained, it can be concluded that the DP value of starting materials (which is much lower for MCC) has no effect on the depth and rate of nitric acid mercerization of cellulose. Sulphite wood cellulose was also noted to be the most active in the traditional mercerization with NaOH solutions (Rydholm, 1965). Similarly, cotton cellulose and Ramie cellulose were classified by Rydholm among the least active.

5 61 Figure 2. Changes in DP of initial (1,2) and microcrystalline (3) wood cellulose during mercerization at 20 C (2,3) and at 0 C (1,3). It is obvious that, on formation of the Knecht compound, the true dependence of the topochemical reaction rate on temperature is reflected in experiments with cotton cellulose, whose degree of chemical purity is superior to that of technological wood cellulose of any production methodology or purification technique. Cellulose contents in the respective raw materials are: up to 98% in cotton seed fluff, and no more than 55% in wood. Due to high contents of lignin and hemicelluloses (including cellulosans) in wood, as well as hard pulping conditions accompanied by destructive transformations, the porous capillary system of cellulose fibres after being isolated and dried becomes clogged, and hence their reactivity decreases. It is known, for example, that dense deposits of low-molecular fractions (consisting mainly of unremoved hemicelluloses) prevent the reagent penetration through the capillary system and make diffusional mechanism the dominant one (Papkov and Fainberg, 1976). That is why pre-activation procedures are of particular importance for chemical transformation of wood cellulose. Cotton cellulose fibres consist almost entirely of α-cellulose, so the impurities produce little effect on their reactivity. The presence of internal channels in the fibres of cotton cellulose facilitate the reagent access to its crystallites. In the case of wood cellulose, to get access to its crystallites, the reagent should first overcome the blocking hemicellulose deposits. As hydrolysing reactivity of an acid decreases on decreasing temperature (Figure 2), one should expect for wood cellulose a much slower reagent diffusion to crystal-

62 Figure 3. Optical microphotographs of flax cellulose (a) and cellulose hydrate (b) fibres. From the bottom upwards: initial fibre; fibre swollen in 68.5% HNO 3 solution; fibre swollen in 18.

6 62 Figure 3. Optical microphotographs of flax cellulose (a) and cellulose hydrate (b) fibres. From the bottom upwards: initial fibre; fibre swollen in 68.5% HNO 3 solution; fibre swollen in 18.5% NaOH solution. lites at lower temperatures compared to the decrease in the capillary transfer which dominates for cotton cellulose fibres. The only way one can note a topochemical transformation is observation of a crystallite scattering in an X-ray pattern. Quite obviously, it is fixed sooner if crystallites are more accessible to the acid molecules. If such access is hampered by the presence of an amorphous component which is invisible in the X-pattern, such fixation is delayed in time. An important fact corroborating this viewpoint is that wood MCC is mercerized at 0 C more rapidly than the initial cellulose. The most probable cause of this acceleration is the weakening influence of the diffusional factor on the process kinetics as a result of the crystallites getting free from their amorphous binding component. The MCC is known to differ from its precursor by not only morphological structure and lower DP, but also by its higher chemical purity (Battista, 1975). In traditional mercerization, the reagent penetration is facilitated because of solubility of hemicelluloses in alkaline media. Moreover, the cellulose swelling in 68 69% HNO 3 is not as pronounced as in NaOH solutions. The microphotographs in Figure 3 allow us to compare alterations in transverse dimensions of cellulose fibres after mercerization by these two different methods. As can be seen, the alkaline medium causes the fibre to become almost twice as thick as that swollen in the acid medium. The incapability of the two cotton cellulose morphological forms to be mercerized completely under conditions providing complete mercerization of the corresponding wood cellulose forms (Figure 1) is in agreement with the idea (Ioelovich, 1991; Ioelovich and Veveris, 1991) of a primordial role played by the crystallite dispersity in the phase transformations associated with the formation of additive compounds. According to literature data (Ioelovich and Veveris, 1985), the crystallites of cotton cellulose ( Hercules

7 brand, L = 10 nm) are considerably larger than the crystallites of wood cellulose ( Florainier brand, L = 6 nm). As a result of hydrolysis of the cellulose fibres down to the limit DP, the L parameter value inevitably increases because of recrystallization and hydrolytic destruction of fractions composed of the smallest crystallites. From analysis of published results (Ioelovich and Veveris, 1983, 1991) it follows that this enlargement is more pronounced in the course of preparation of cotton MCC than that for wood MCC ( 35% against 20%). This increasing difference in mean crystallite size between the two MCC species (that obtained from cotton cellulose and that obtained from wood cellulose) causes a still more pronounced difference in the degree of mercerization which may attain a factor of 10 (Figure 1b). In spite of the fact that the mean crystallite size value L increases as a result of partial hydrolysis, the wood MCC is mercerized at 0 C more rapidly than the initial cellulose. However, the crystallite growth leads to the appearance of a well-defined slow (final) stage of mercerization of the largest crystallites. The crystallite dimensions of the two morphological forms of wood cellulose are likely to be below the level which restricts the depth of polymorphic transformation under the conditions considered. It would be quite reasonable to assume that this very circumstance causes wood cellulose to be more suitable for the nitric acid mercerization than the cotton one. In the case of traditional mercerization, the correlation between the depth of polymorphic transformation and the crystallite dimensions is disrupted when the cellulose is present in different morphological forms. Cotton MCC, having the largest crystallites ( L = 13.5 nm, Ioelovich and Veveris, 1985, 1991), is mercerized significantly better than its fibrous percursor ( Hercules brand cellulose, L = 10 nm). A 1/2 h interaction of the former with 19% NaOH solution at 20 C provides its complete polymorphic transformation. whereas the transformation of the initial Hercules brand cellulose remains only partial even after a 1-h soaking in this solution. It seems that under conditions of extensive swelling typical for conventional mercerization, the role of the DP value becomes more important, and this parameter is 10 times lower for Hercules brand MCC compared to that for the initial cellulose. During the nitric acid mercerization, the conventional (amorphocrystalline) cellulose undergoes a considerable depolymerization. Kinetic curves showing the decrease of DP (Figure 2) are typical for heterogeneous reactions of cellulose hydrolysis. A rapid decrease in DP of the initial cellulose at the beginning of its interaction with 68.5% HNO 3 is followed by a slow degradation stage. In the first (rapid) stage, the rate of hydrolytic cleavage at 20 C is 4 5 times higher than at 0 C. The complete polymorphic transformation of wood cellulose which occurs during a 1-h interaction with HNO 3 at 20 C is accompanied by a 3-fold decrease in DP compared to its 63

8 64 initial value of 1200, whereas an only 1.5-fold DP decrease is observed at 0 C, the reaction with HNO 3 being completed in 3 h. In contrast to initial cellulose, the respective MCC remains stable towards hydrolysis even under conditions of intracrystallite swelling. As a probable factor preventing the destructive processes, the high stoichiometry of interaction between cellulose macromolecules organized in crystallites and 68.5% HNO 3 can be pointed out (Gess, 1928). According to the data available in the literature (Andress, 1928), the stoichiometric ratio of HNO 3 molecules to anhydroglucose units of cellulose in the Knecht compound is 1:2. During mercerization of wood MCC at both 20 Cand0 C (completed within h of interaction with HNO 3 ), the decrease in DP compared to the initial value of 160 does not exceed 30 units (Figure 2, curve 3). There are no reasons to question suitability of the technique employed for determination of both high and low DP values of cellulose. When wood cellulose is subjected to mercerization with 68.5% HNO 3 followed by hydrolysis with boiling diluted HNO 3, the above-mentioned technique gives the DP value of 50 for the MCC- II obtained in this way. This result is in good agreement with the DP values reported by Battista (1975) for microcrystalline cellulose hydrate prepared from various starting materials. There were no apparent signs of oxidative processes in the nitric acid mercerization of cellulose at room temperature; at the same time, it was noted that a partial esterification does occur (Gert, 1996). The accumulation rate of bound nitrogen depends strongly on the temperature at which the reaction with HNO 3 is carried out (Figure 4). The morphology of starting material does not affect the esterification process to an appreciable extent. Common wood cellulose accumulates approximately equal amounts of nitrogen, % (DS ONO2 = 0.08 to 0.09), during complete mercerization either at 20 C (1 h interaction with HNO 3 )or0 C (3 h interaction with HNO 3 ). In the case of wood MCC, the influence of the nitration reaction on the crystalline structure of the products becomes appreciable. We noticed that the MCC mercerized at 0 C (2.5 h interaction with HNO 3 ) is significantly superior in crystallinity to the MCC mercerized at 20 C (2 h interaction with HNO 3 ) (Figure 5). The latter one is notable for its two-fold higher bound nitrogen content ( 1.3%; DS ONO2 = 0.15). If the mercerization time is increased up to 4 h, the amount of bound nitrogen rises to 2% (DS ONO2 = 0.25). In X-ray diffractograms of the products, a gradual degeneration of crystallite scattering with increasing degree of esterification is observed (Figure 5, curve 3). At the same time, in X-ray diffractogram of the product of MCC mercerization at 0 C during 4 h, a quite distinct crystallite scattering of cellulose-ii is observed (Figure 5, curve 2), since the bound nitrogen content in this case does not exceed 0.9% (DS ONO2 = 0.1). An obvious correlation

9 65 Figure 4. Amounts of nitrogen chemically bound to initial ( ) and microcrystalline ( ) wood cellulose as function of time of interaction with 68.5% HNO 3 at 20 C(1)andat0 C(2). Figure 5. X-ray diffractograms of wood microcrystalline cellulose (1) and products of its mercerization during 4 h at 0 C (2), and during 3 h at 20 C(3).

10 66 can be discovered between the degree of esterification and the degree of decrystallization of the mercerized MCC. We think the main cause of structural disorganization of MCC during its nitric acid mercerization is associated with its partial esterification under conditions of intracrystallite swelling. It should be emphasized that a well-defined decrystallization effect can only be observed in the case of wood MCC. In our opinion, its manifestation is favoured by the small chain length of MCC molecules, which is comparable in size with a segment of ordinary cellulose. References Andress, K. R. (1928) About the action of moderately concentrated nitric acid on cellulose (Ger.). Z. Phys. Chem. 136, Battista, O. A. (1975) Microcrystal Polymer Science. New York, McGraw-Hill. Cheronis, N. D. and Ma, T. S. (1964) Organic Functional Group Analysis by Micro and Semimicro Methods. New York, Interscience. Gert, E. V., Shishonok, M. V., Torgashov, V. I. and Kaputskii, F. N. (1990) Crystallization of amorphous cellulose in the form of inclusion compounds. J. Polym. Sci. Part C: Polym. Lett. 28, Gert, E. V. (1996) Possibilities of nitric acid preparation of powder cellulose forms. Cellulose 3, Gert, E. V. (1997) Labile products of interaction of cellulose with nitrogen oxide compounds. Russian Chem. Rev. 66, Gess, K. (1928) Chemistry of cellulose and concomitant substances (Ger.). Leipzig, Akademische Verlagsgesellschaft m.b.h. Ioelovich, M. Ya. and Veveris, G. P. (1983) Cellulose-II content determination by X-ray internal standard method (Russ.), Khimiya drevesiny 2, Ioelovich, M. Ya. and Veveris, G. P. (1985) Studies on sizes and defects of crystalline areas in cellulose (Russ.), Khimiya drevesiny 6, Ioelovich, M. Ya. (1991) Effects of dispersity of cellulose crystalline areas on their phase transitions under the action of hydroxide solutions (Russ.). In Delignification and Cellulose Chemistry. Riga, Zinatne Publ., pp Ioelovich, M. Ya. and Veveris, G. P. (1991) Investigation of regularities in phase transitions of crystalline areas in cellulose under the action of nitric acid solutions (Russ.). In Delignification and Cellulose Chemistry. Riga, Zinatne Publ., pp Knecht, E. (1904) About a labile cellulose nitrate (Ger.), Ber. 37, Nikitin, N. I. (1962) Wood and cellulose chemistry (Russ.). Moscow-Leningrad, USSR Acad. Sci. Publ. Papkov, S. P. and Fainberg, E. Z. (1976) Interaction of cellulose and cellulosic materials with water (Russ.). Moscow, Khimiya Publ. Rydholm, S. A. (1965) Pulping processes. Interscience Publ. Warwicker, J. O. (1974) Swelling (Russ.). In Cellulose and Cellulose Derivatives. N. M. Bikales and L. Segal (eds) vol. 1. Moscow, Mir Publ., pp

Study of Cellulose Interaction with Various Liquids

Study of Cellulose Interaction with Various Liquids Michael Ioelovich Designer Energy Ltd, Bergman Str, Rehovot, Israel * Corresponding author: Michael Ioelovich, Designer Energy Ltd, Bergman Str, Rehovot,