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Weight loss zero-order

Polysaccharide pyrolysis at 375-520°C is accompanied by a higher rate of weight loss and evolution of a complex mixture of vapor-phase compounds preponderantly of HsO, CO, C02, levoglucosan, furans, lactones, and phenols (Shafizadeh, 1968). The volatile and involatile phase compositions are conditional on the rate of removal of the vapor phase from the heated chamber (Irwin, 1979), inasmuch as the primary decomposition products are themselves secondary reactants. The reaction kinetics is described as pseudo zero order (Tang and Neill, 1964) and zero order initially, followed by pseudo first order and first order (Lipska and Parker, 1966), suggesting an... [Pg.119]

Figures 4A-D show that most of the pulps have initial kinetics approaching zero order. Pulps FB, SBS, BBC, and BRK all show two-component curves with a minor peak indicated between 0.05 and 0.1 conversion, possibly corresponding to the hemicellulose fraction as discussed earlier. The maximum rate of weight loss occurs for all pulps at about 60-65% loss of initial weight. This corresponds to plots of theoretical rate of weight loss vs. conversion plots of reaction order Vi to 1(14). Figures 4A-D show that most of the pulps have initial kinetics approaching zero order. Pulps FB, SBS, BBC, and BRK all show two-component curves with a minor peak indicated between 0.05 and 0.1 conversion, possibly corresponding to the hemicellulose fraction as discussed earlier. The maximum rate of weight loss occurs for all pulps at about 60-65% loss of initial weight. This corresponds to plots of theoretical rate of weight loss vs. conversion plots of reaction order Vi to 1(14).
Golova and Krylova pyrolyzed cotton cellulose and measured the decrease in the D-glucose residues, instead of the weight-loss, as a function of time, and found that the reaction follows a zero order. In contrast, Tang and Neill, on the basis of thermogravimetric data to be discussed later (see p. 446), suggested that the initial state of pyrolysis of cellulose is controlled by pseudo-zero-order kinetics and the final state is of pseudo-first order. [Pg.444]

Cellulose has also been pyrolyzed at 250-298° in a fluidized bed in a nitrogen atmosphere. The results indicated that, at any temperature, an initial, rapid decomposition and weight-loss were followed by a zero-order reaction, and then the decomposition became first-order. [Pg.511]

When a water-insoluble drug such as hydrocortisone is physically dispersed in the polymer and the polymer fabricated into thin disks which are then placed in a buffer at pH 7.4, release kinetics shown in Fig. 1 are obtained. Clearly, this system exhibits excellent zero order drug release kinetics with concomitant linear rate of polymer weight loss. The simultaneous polymer erosion and drug release is a clear indication that erosion occurs at the surface of the device and that drug release is controlled by erosion of the polymer. [Pg.43]

Studies on the thermal degradation of PE samples with different molecular masses in the isothermal regime at different temperatures have shown that the kinetic curves have linear plots up to 70% weight loss (Figure 1.2), which point to a zero-order reaction. The activation energy of thermal degradation increases with the molecular mass of the polymer from 192.3 kj/mol (molecular mass 11,000) up to 276.3 kj/mol (molecular mass 23,000) [2]. [Pg.6]

The effect of co-PEA composition on enzymatic biodegradation was studied in enzyme phosphate buffer solution. Figure 2 summarizes lipase-catalyzed weight loss data from four different types of co-PEA films a) 4-Leu(6)o.75-Lys(Bz)o.25, b) 8-Leu(6)o.75-Lys(Bz)o.25, c) 4-Leu-(6)o.5-Phe(6)o.25-Lys(Bz)o.25, d) 8-Leu(6)o.5-Phe(6)o.25 Lys(Bz)o.25 There was virtually no weight loss in the PBS control during the same testing period. Several of these co-PEAs show close to zero order kinetics, a property important for sustained and controlled release of drugs. [Pg.17]

Whatever the explanation, there is no doubt that polymer of molecular weight 500—800 is formed extremely rapidly at the start of polymerization and can be isolated from the final product. With fluorenyllithium [168], (toluene—ether, —60°C) a first order disappearance of monomer is observed, which extrapolates at zero time, not to the original added monomer concentration but to a concentration corresponding to the immediate loss of three molecules of monomer per initiator molecule. With 1,1-diphenylhexyllithium [174] (toluene,—30°C) this extrapolation corresponds to the rapid addition to the initiator of about five monomer units. In this case termination at various times and isolation of precipitant-soluble material confirms that polymer of molecular weight 830 is formed rapidly and does not change appreciably in amount throughout the polymerization. With butyllithium [173] (toluene, —30°C) the course of reaction is more complex in the initial stages but eventually a steady concentration of active centres is probably formed as the reaction settles down to first order decay in monomer. A second addition of monomer at the end of the reaction then produces a first order disappearance of monomer immediately. The two first order rate coefficients are identical. Evidently products are produced with butyl-lithium which disturb the reaction, and until these are removed a steady concentration of active centres is not achieved. [Pg.43]


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