on the thermal degradation of cellulose allomorphs

CELLULOSE CHEMISTRY AND TECHNOLOGY

ON THE THERMAL DEGRADATION OF CELLULOSE ALLOMORPHS DIANA CIOLACU and VALENTIN I. POPA* “Petru Poni” Institute of Macromolecular Chemistry, 700487 Iasi, Romania *“Gh. Asachi” Technical University, 700050 Iasi, Romania

Received October 21, 2005 The complexity of cellulose thermal degradation results from the large number of parallel and consecutives reaction steps, from the change of the predominant reaction route with degradation temperature, from the intense influence of the ambient atmosphere on the degradation process and, last but not least, from the important role played by the structure of cellulose sample in thermal degradation. To determine the effect of the supramolecular structure on cellulose thermal decomposition, three crystalline forms of cellulose – I, II and III, respectively – have been prepared. For a systematic analysis of the thermal behaviour of cellulose allomorphs, cotton cellulose was used. The thermal properties of cellulose allomorphs were investigated by TG, DTG and DTA techniques. The study shows that the crystal form, as well as the crystallinity index, can affect the thermal stability of cellulose allomorphs; it appears that the thermal stability of cellulose III crystal form is lower than those of cellulose I or II.

Keywords: cellulose allomorphs, crystallinity, thermogravimetry, differential thermogravimetry INTRODUCTION All theories on cellulose structure assume the existence of two distinctly different regions: the crystalline domains – where an extensive intermolecular hydrogen bonding is observed and the molecules are densely packed in an ordered fashion, and the amorphous area – where the molecular arrangement is less ordered and compact.1 The crystalline domains, characterized by strength, swelling resistance, determine fibers stability. Due to a higher accessibility of the amorphous regions, degradative processes of cellulose tend to occur predominantly in such areas. The thermal decomposition of cellulose and cellulose derivatives has been extensively investigated. Thermal analysis may be defined as a set of techniques (DTA, DSC and TG) used to describe the physical or chemical changes associated to substances as a function of temperature.2 Changes in latent heat in a chemical substance are detectable by differential scanning calorimetry (DSC), while the progressive

mass loss is detectable by thermogravimetry (TG) or differential thermogravimetry (DTG).3 The thermal decomposition of cellulose involves at least four processes in addition to the simple desorption of physically bound water, namely: 4-6 - crosslinking of cellulose chains, with the evolution of water (dehydration); unzipping of cellulose chain and levoglucosan formation from the monomeric unit; - decomposition of the dehydrated products to yield char and volatile products; - decomposition of levoglucosan to yield smaller volatile products, including tars and, eventually, carbon monoxide. Cellulose polymorphism has always been an intriguing matter as for the unelucidated aspects regarding both the formation and structural organization of the allomorphic forms and their behaviour during thermal treatments. To elucidate the thermal behaviour of cellulose allomorphs

Cellulose Chem. Technol., 40 (6), 445-449 (2006)

Diana Ciolacu and Valentin I. Popa

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Methods The degree of polymerization of cellulose (DP) was measured by viscometry in 0.5 mol Cuen.7 X-ray diffraction method - X-ray diffraction patterns of the samples were collected on a RIGAKU RINT 2500 apparatus, equipped with a transmission type goniometer, using nickelfiltered, CuKα radiation at 40 kV. The goniometer was scanned stepwise every 0.10° from 10 to 40° in the 2θ range. The resulting diffraction patterns exhibited peaks deconvoluted from a background scattering by using Lorenzian functions, while the diffraction pattern of an artificially-amorphicized sample was approximated by a Gaussian functions curve fitting analysis.8 Estimation of the crystallinity index of the cellulose samples was established9 from the ratio between the crystallinity area (SC) and the total area (ST) - Cr.I.1, as well as by the Segal method (Cr.I.2). Thermogravimetry - The thermo-destructive behavior of the cellulose samples was evaluated on an Erdey, Paulik & Paulik MOM Budapest Derivatograph. The analysis was run in air atmosphere, on samples (5 mg) placed in platinum holders, at a heating rate of 10 °C/min. Differential thermal analysis – The analysis was run on 5 mg samples, in air atmosphere, at a heating rate of 10 °C/min on an Erdey, Paulik &

RESULTS AND DISCUSSION Characterization of the allomorphic forms of cotton cellulose by X-ray diffraction represents the only physical method that allows the precise determination of the polymorphous modifications the unit cell goes through.10 Every crystalline form of the cellulose organization displays a characteristic difractogram by variations in diffraction intensities and the maximum values of the Bragg angles (Figs. 1-3).

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EXPERIMENTAL Materials Cotton cellulose - Pakistan cotton (DP = 3078) - (BI), cellulose sample with crystalline form of cellulose I, has been used. The cotton cellulose was extracted in a Soxhlet extractor with ethanol/benzene for 8 hours, then boiled in a 1% NaOH solution for 6 hours, washed with distilled water, neutralized in 1% acetic acid, washed with water, and finally dried in air. Characteristic crystalline forms of cellulose allomorphs II and III, were obtained. Cellulose II was prepared from cotton cellulose by soaking it in 17.5% NaOH solution for 24 hours at 15 °C, followed by washing thoroughly with distilled water and air drying. Cellulose III was obtained by soaking the cotton cellulose in organic amine (100% ethylenediamine) for 24 hours at room temperature. The cellulose amine complex was washed with anhydrous methanol and finally airdried.

Paulik MOM Budapest Derivatograph, with alumina as reference material.

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and to determine the effect of the supramolecular structure on the thermal decomposition of cellulose allomorphs, thermal analytical techniques, i.e., TG, DTG and DTA, were used.

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Figure 2: X-ray diffractogram of cellulose II obtained from cotton cellulose (BII)

Thus, in the pattern of cellulose I diffractogram, the (101) and ( 10 1 ) planes have almost the same height, while the (002) plane presents a higher height of the peak at a height Bragg angle. For cellulose II, a movement of the ( 10 1 ) plane near the (002) planes may be observed while, for cellulose III, these planes are overlapping and the (101) plane appears at a smaller Bragg angle. The Bragg angles and the peak areas of the lattice planes characteristic to the allomorphs obtained from cotton cellulose are presented in Table 1.

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Modification of the crystalline organization form of cellulose through different chemical treatments unavoidably takes place, while the index of crystallinity goes down (Table 2). The order of the crystallinity indexes of the polymorphs forms thus obtained is cellulose I > cellulose II > cellulose III.

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Figure 3: X-ray diffractogram of cellulose III obtained from cotton cellulose (BIII) Table 1 X-ray diffraction angles of cellulose allomorphs Cellulose allomorph Cellulose I Cellulose II Cellulose III

(101) 15.07 12.62 12.17

2θ, ° ( 10 1 ) 16.77 20.31 21.12

Table 2 Crystallinity indexes of cellulose allomorphs from cotton cellulose Crystallinity index, % Cellulose allomorph Cr.I.1a) Cr.I.2b) Cellulose I 71.11 83.33 Cellulose II 60.57 79.16 Cellulose III 53.23 73.02 a) determined by area method b) determined by Segal method

The behaviour of the polymorphic forms of cellulose during the thermooxidative destruction was evaluated by thermogravimetric studies. The process of thermal destruction takes place more rapidly for the samples with a higher content of domains with high organization levels, which can be explained from the derivative thermogravimetric curves, by the higher rate at which mass is being lost (Fig. 4).11,12 Reduction in the crystallinity index of the polymorphs of cotton cellulose leads to a shift in the maximal temperature of the main step of decomposition towards smaller values, which indicates that the more crystalline samples are those more stable during thermal destruction. In the present case, the shift in the maximal temperature of the corresponding peak of decomposition was evidenced in the

(002) 22.93 22.23 21.12

(101) 15.85 7.33 8.27

Peak area, % ( 10 1 ) 17.92 36.15 32.27

(002) 49.31 35.66 32.27

following order: B I > B II > B III, which shows that cellulose III has the lowest thermal stability, but also the lowest crystallinity.

Figure 4: Thermal behaviour (DTG) of allomorphs of cotton cellulose: 1 – BI, 2 – BII, 3 – BIII

The most stable allomorphic form is cellulose II (Fig. 5), which can be explained by the presence of a compact network of hydrogen bonds in the supramolecular structure of cellulose II, as a consequence of the mercerization process performed in 3 alkaline solution.1 To evaluate the influence of the form of crystalline organization and of the type of cellulose allomorph, respectively, during the thermal degradation reaction, the kinetic 447

Diana Ciolacu and Valentin I. Popa parameters were determined by the Freemen – Carroll method (Table 3).

indicated that cellulose I is the most thermally stable allomorph (Fig. 6). Table 4 Thermal analytical data corresponding to the allomorphs of cotton cellulose Sample Ti, °C Tm, °C Cellulose I 323 340 Cellulose II 281 315 Cellulose III 280 309

Figure 5: Thermal behaviour (TG) of allomorphs of cotton cellulose: 1 – BI, 2 – BII, 3 – BIII Table 3 Thermal analytical data corresponding to the allomorphs of cotton cellulose Sample Ti, Tm, Tf, Ea, n R, °C °C °C kJ/mol·K % BI 291 335 373 202.54 1.0 8.36 BII 281 328 363 190.64 0.8 10.62 BIII 263 312 345 178.85 1.0 10.59

The residue obtained after the thermal degradation of cellulose cotton allomorphs is an expression of sample crystallinity index.14 Lowering of crystallinity determines a decrease of the activation energy and an increase of residue amount. Analysis of the DTA curves reflects characteristic modifications in the morphological structure specific to each type of cellulose allomorph. The main thermal characteristics (onset temperature – Ti, and peak temperature – Tm) for cellulose allomorphs are affected both by the transformation of the cell form and by the crystallinity index (Table 4). Increase in the amorphous percentage of the cellulose sample is reflected in the DTA curve by the strong decrease of the endothermic peak characteristic to the main step of thermal destruction. Shifting from allomorph I to those of cellulose II and cellulose III, respectively, a modification in the values of maximum temperature, corresponding to the endothermic peak, in the sense of its shift toward lower values, was noticed, which 448

Figure 6: Thermal behaviour (DTA) of allomorph forms of cotton cellulose: 1 – BI, 2 – BII, 3 – BIII

CONCLUSIONS The study was mainly focused on the behavior of the polymorphic forms of cellulose during thermo-oxidative destruction. The experimental results showed that thermal destruction takes place more rapidly in the domains with higher organization levels. Decrease in the crystallinity index of cellulose allomorphs determines a decrease of the activation energy and an increase in residue amount, respectively. Also, diminishing the crystallinity of cellulose polymorphs leads to a shift in the maximal temperature of the main step of decomposition towards smaller values. Cellulose III, with the lowest crystallinity, presents the lowest thermal stability. An increase in the amorphous percentage of cellulose allomorphs determines a shift of the endothermic peaks in DTA curve to lower values. REFERENCES 1

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