Cellulose activation energy

Activation energy Activation of cellulose Activation parameters Activation volume Activators... 

Lu and Pizzi [83] showed that lignocellulosic substrates have a distinct influence on the hardening behavior of PF-resins, whereby the activation energy of the hardening process is much lower than for the resin alone [84]. The reason is a catalytic activation of the PF-condensation by carbohydrates like crystalline and amorphous cellulose and hemicellulose. Covalent bonding between the PF-resin and the wood, especially lignin, does not play any role [84]. 

This conclusion is in agreement with experiments in which a smootb quartz and cellulose were used as substrates. For above materials the transfer of excitation energy of the dye into the substrate is low which is confirmed by intensive luminescence of adsorbed tripaflavine. Note, that the activation energy of emission of singlet oxygen is close for zinc oxide oxidized by oxygen atoms, quartz and cellulose and amounts to 5-10 kcal/mol [83]. 

In cellulose II with a chain modulus of 88 GPa the likely shear planes are the 110 and 020 lattice planes, both with a spacing of dc=0.41 nm [26]. The periodic spacing of the force centres in the shear direction along the chain axis is the distance between the interchain hydrogen bonds p=c/2=0.51 nm (c chain axis). There are four monomers in the unit cell with a volume Vcen=68-10-30 m3. The activation energy for creep of rayon yarns has been determined by Halsey et al. [37]. They found at a relative humidity (RH) of 57% that Wa=86.6 kj mole-1, at an RH of 4% Wa =97.5 kj mole 1 and at an RH of <0.5% Wa= 102.5 kj mole-1. Extrapolation to an RH of 65% gives Wa=86 kj mole-1 (the molar volume of cellulose taken by Halsey in his model for creep is equal to the volume of the unit cell instead of one fourth thereof). 

In this study five cellulose samples of different crystallinities (10, 41, 63, 67, and 742) were treated to 10% by weight with H PO, H3BO3, and AlClo i O. These treated samples and untreated (control) samples were isothermally pyrolyzed under N2 at selected temperatures and the TGA data analyzed by four methods (0—, 1st-, and 2nd-order and Wilkinson s approximation) to obtain rates of mass loss. From these rates, activation energy (Efl), activation entropy (AS+) and enthalpy (AH+) values were obtained. Efl was also determined by the integral conversion method. 

Thermogravimetric analysis (TGA) has often been used to determine pyrolysis rates and activation energies (Ea). The technique is relatively fast, simple and convenient, and many experimental variables can be quickly examined. However for cellulose, as with most polymers, the kinetics of mass loss can be extremely complex (8 ) and isothermal experiments are often needed to separate and identify temperature effects (9. Also, the rate of mass loss should not be assumed to be related to the pyrolysis kinetic rate ( 6 ) since multiple competing reactions which result in different mass losses occur. Finally, kinetic rate values obtained from TGA can be dependent on the technique used to analyze the data. 

Such activation energies of abiotic heat release for lignocellu-losic material have recently been summarized and and compared in detail by Kubler (14). These activation energies in the range from 120°C to 220°C, when air is present, usually are between 60 and 115 kJ/mole, i.e. 15 to 30 kcal/mole (14). Activation energies for the delignified cellulose-hemicellulose material presented here are at the lower end of this range. 

The influence of substituents on the rates of degradation of arylazo reactive dyes based on H acid, caused by the action of hydrogen peroxide in aqueous solution and on cellulose, has been investigated [43]. The results suggested that the oxidative mechanism involves attack of the dissociated form of the o-hydroxyazo grouping by the perhydroxyl radical ion [ OOH]. The mechanism of oxidation of sulphonated amino- and hydroxyarylazo dyes in sodium percarbonate solution at pH 10.6 and various temperatures has also been examined. The initial rate and apparent activation energy of these reactions were determined. The ketohydrazone form of such dyes is more susceptible to attack than the hydroxyazo tautomer [44]. 

Both Tg and the activation energy below Tg Increase with cation size (Table VI). The activation energy values for p-Cl-PHMP compare favorably with the results of 1on conductivity measurements 1n cellulose acetate (16.18). shown 1n Figure 8. This relationship speaks strongly for cation diffusion as being Involved in the rate determining step. 

Figure 8. Activation energies of p-Cl-PHMP dissolution in different alkalis ( ) and ionic conductivities in cellulose acetate ( ).

Typical values given in the literature for Lyocell dopes without stabilizers added are DP losses from 472 to 177 over 6 hours at 105 °C [22], or from 570 to 185 over 2 hours at 90 °C [23]. The activation energy for the cellulose chain cleavage in NMMO at temperatures above 115 °C was determined to be 69 kj/mol. This observation strongly supported the hypothesis that the actual rate-determining step of the cellulose chain cleavage was a /1-alkoxy elimination, with an activation energy of 67-72 kj/mol [24,25]. 

Table VII shows that the activation energies of artificially aged pulps BBC and BPC are consistently lower than for the control samples. The lower values indicate that the rate-determining reaction can proceed more readily after artificially aging. This may be a result of the introduction of carbonyl groups or of the introduction of chain ends as a result of hydrolysis of the hemicellulose and cellulose. Such modifications of the original pulp may act as weak links or sites at which further degradation is facilitated.

Graft polymerization of 2-methyl-5-vinyltetrazole onto cellulose fiber decreased the temperature for initial oxidative thermal degradation and increased the activation energy of degradation compared with that for the initial cellulose... 

Acid hydrolysis, regenerated cellulose, 341/ Activation energies, warp and weft yams, 68 ... 

Stamm has determined the rates and activation energies for thermal degradation of a-cellulose, cotton, rayon, and paper, all of which were found to follow a first-order reaction. A summary of the results obtained on the basis of physical properties is given in Table III. 

Following this lead, Martin and coworkers proceeded with acquisition of further data to clarify the ignition behavior of cellulose, but they rejected the use of ElR as being artificial because of the substantial variation in the overall weight-loss activation energy among the cellulosic materials. However, they found that the hypothetical, heat-flow model serves as a convenient device for correlation of their experimental results and the interpretation of the ignition behavior. 

Extensive kinetic investigations have also indicated that the activation energy of the overall thermal-decomposition process is substantially lowered by the addition of sodium chloride and sodium carbonate. Madorsky and coworkers have, therefore, proposed that these salts catalyze the dehydration of cellulose by scission of the C —O bonds (bonds a, b, and c in 1 see p. 438), and that this results in destruction of the hexose units and increases the yield of water and char at the expense of levoglucosan. This theory has found substantial support in subsequent experiments and publications however, it may be noted here that Golova and associates" consider that inorganic salts promote the cleavage of C—C, rather than C—O, bonds in the macromolecule. 

TABLE 13.3 Apparent Activation Energies of Product Gases from Short-Residence-Time Cellulose Pyrolysis at 500 to 675°C ... 

---